Influenza, commonly known as the flu, is a contagious illness caused by influenza viruses. These viruses mainly affect the nose, throat, and lungs. The flu occurs worldwide and can affect people of all ages. The severity ranges from mild cases that improve on their own to serious illness that may require hospital care or, in some cases, lead to death.
Common symptoms include sudden fever, cough, sore throat, runny or blocked nose, body and muscle aches, tiredness, and headache. Some people, especially children, may also experience stomach-related symptoms such as nausea, vomiting, or diarrhea. Symptoms usually appear one to four days after infection and last about a week, although tiredness and cough can last longer.
The most effective way to prevent the flu is through yearly vaccination. Vaccines are supported by simple protective measures such as frequent handwashing, covering coughs and sneezes, and staying home when sick to avoid spreading the virus.
Influenza viruses change often through natural genetic changes. Small, gradual changes are called antigenic drift, while larger, less common changes are known as antigenic shift. These changes lead to new flu strains each year and, in rare cases, global outbreaks known as pandemics. Because of this, flu vaccines are updated annually to match the strains most likely to circulate.
Types of Influenza Viruses
Influenza A
Influenza A viruses are the most variable and are responsible for both seasonal epidemics and global pandemics. They infect humans as well as many animals, including birds, pigs, horses, and marine mammals. Birds, especially wild waterfowl, act as the primary natural reservoir for most influenza A strains.
These viruses are further divided into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N). Hemagglutinin allows the virus to attach to and enter host cells, while neuraminidase helps release newly formed viral particles from infected cells. Different combinations of these proteins define subtypes such as H1N1, H3N2, and H5N1. To date, scientists have identified 18 hemagglutinin subtypes and 11 neuraminidase subtypes.
Among these, H1N1 and H3N2 are the main subtypes that circulate widely in humans, causing regular seasonal flu outbreaks. Others, such as H5N1 and H7N9, are primarily found in birds but can occasionally infect humans, sometimes resulting in severe and life-threatening illness.
Influenza B
Influenza B viruses also cause seasonal epidemics but are generally less variable than influenza A. Unlike influenza A, they primarily infect humans and, to a lesser degree, seals.
Instead of subtypes, influenza B viruses are divided into two major lineages: B/Yamagata and B/Victoria. Both lineages circulate in humans and are typically included in seasonal influenza vaccines to provide broad protection.
Although influenza B infections can still result in significant illness, they are not linked to pandemics and are considered less likely to cause widespread outbreaks compared to influenza A.
Influenza C
Influenza C viruses are associated with milder respiratory illness than types A and B. Infections are usually limited to the upper respiratory tract, producing cold-like symptoms rather than severe flu-like disease.
These infections occur more frequently in children than in adults and generally do not pose a major public health concern. Influenza C does not cause epidemics, but outbreaks can occasionally occur in schools or other closed communities.
Influenza D
Influenza D viruses are primarily found in cattle and have also been detected in pigs. Unlike types A, B, and C, influenza D has not been shown to infect or spread among humans.
Its main significance lies in veterinary medicine, where it is studied for its role in respiratory diseases of livestock. Because it does not infect people, influenza D is not considered a human health threat.
Key Differences
Types A and B are the main causes of seasonal flu epidemics, with type A capable of producing global pandemics due to its ability to jump between species and undergo major genetic shifts. Type B remains confined to humans and does not cause pandemics.
In terms of severity, type A infections can be serious across different species, while type B generally causes moderate disease in humans. Type C typically results in mild respiratory illness, and type D is restricted to animals.
Regarding diversity, type A shows the greatest variation, with multiple subtypes that change regularly. Type B has two main lineages, while types C and D are more genetically stable and less diverse.
History of Influenza
Accounts of influenza-like illness have been described for centuries, with some historians tracing references back to ancient Greece and the writings of Hippocrates around 412 BCE. However, the first detailed descriptions resembling modern influenza were recorded in the 16th century, particularly during outbreaks that spread rapidly across Europe and Asia. The term influenza originates from Italian, meaning “influence,” reflecting the historical belief that the disease was caused by the influence of the stars.
Over time, influenza became recognized as a recurrent epidemic disease, with major outbreaks occurring every few decades. Improved record-keeping in the modern era made it possible to distinguish influenza from other respiratory illnesses, and by the late 19th century, physicians began recognizing it as a distinct viral infection. In 1933, British researchers isolated the first influenza virus (influenza A), marking a turning point in scientific understanding and enabling the later development of vaccines.
The most devastating influenza pandemic in recorded history was the 1918 influenza pandemic, commonly known as the Spanish flu. Caused by an H1N1 influenza A virus, it spread worldwide in multiple waves and infected an estimated one-third of the global population. Death toll estimates range from 20 million to 50 million, with some analyses suggesting the number may have been as high as 100 million. Unlike typical influenza seasons, the 1918 pandemic disproportionately affected young, healthy adults in addition to vulnerable groups.
Subsequent pandemics further demonstrated the virus’s ability to mutate and cross species barriers. The 1957 Asian flu, caused by an H2N2 virus, originated in East Asia and spread globally, leading to an estimated 1–2 million deaths. In 1968, the Hong Kong flu, caused by an H3N2 virus, emerged and resulted in around 1 million deaths worldwide, though it was less severe than the 1918 and 1957 pandemics.
More recently, the 2009 H1N1 pandemic, also known as the swine flu, originated in North America and spread rapidly across the globe. This strain was a novel reassortment of influenza viruses from pigs, birds, and humans. While the 2009 pandemic was relatively mild compared to earlier ones, it still caused widespread illness and led to an estimated 150,000–575,000 deaths worldwide in its first year.
Advancements in medical science, including the development of vaccines, antiviral drugs, and global surveillance systems, have greatly improved the ability to manage and mitigate influenza outbreaks. Annual influenza vaccination programs began in the mid-20th century and remain the most effective tool for preventing widespread illness. Antiviral medications, first introduced in the 1990s, have provided additional treatment options for reducing disease severity and complications.
Epidemiology
Seasonal patterns of influenza vary by climate. In temperate regions, influenza activity typically peaks during the winter months, coinciding with colder weather and greater indoor crowding, which facilitates viral transmission. In tropical and subtropical regions, influenza occurs throughout the year, with peaks often linked to the rainy season, though multiple smaller waves may be observed. These differences in seasonality complicate global vaccine timing and distribution strategies.
Globally, seasonal influenza causes significant health and economic burdens. The World Health Organization (WHO) estimates that annual influenza epidemics result in 3 to 5 million cases of severe illness and 290,000 to 650,000 respiratory deaths worldwide. The burden is unevenly distributed, with higher mortality rates reported in low- and middle-income countries, where access to vaccines, healthcare facilities, and antivirals may be limited.
Certain groups are at greater risk of severe illness and complications from influenza. These include young children, especially those under five years old; older adults over the age of 65; pregnant women; and individuals with underlying medical conditions such as asthma, diabetes, cardiovascular disease, or weakened immune systems. Influenza can also increase the risk of secondary bacterial infections, such as pneumonia, which further contributes to morbidity and mortality in vulnerable populations.
Vaccination remains the cornerstone of influenza prevention. Because influenza viruses undergo frequent genetic changes, annual vaccination is recommended. Seasonal influenza vaccines are formulated based on global surveillance data coordinated by the WHO Global Influenza Surveillance and Response System (GISRS). This network monitors circulating strains worldwide and helps predict which strains are most likely to dominate in upcoming flu seasons, guiding vaccine composition.
Symptoms of Influenza
Influenza presents with a range of symptoms that affect the respiratory system and general well-being. These symptoms can vary in intensity depending on factors such as age, health status, and virus strain.
Common Symptoms
Influenza typically causes a sudden onset of fever, often above 100.4°F (38°C). A cough, sore throat, and runny or stuffy nose are frequent. Muscle aches, headaches, and fatigue also commonly occur.
Chills and sweating can accompany the fever. Gastrointestinal symptoms like nausea or diarrhea are less common but more frequent in children. Symptoms usually last about 3 to 7 days, with some fatigue persisting longer.
Severe Complications
While most healthy individuals recover from influenza within one to two weeks, the infection can lead to significant complications, particularly in vulnerable populations. Older adults, infants, pregnant women, and people with weakened immune systems or chronic medical conditions such as asthma, diabetes, chronic obstructive pulmonary disease (COPD), or cardiovascular disease face heightened risks.
One of the most common and serious complications is pneumonia, which may be primary viral pneumonia directly caused by the influenza virus or secondary bacterial pneumonia, often due to pathogens such as Streptococcus pneumoniae or Staphylococcus aureus. Secondary pneumonia can be more severe and often requires hospitalization.
Influenza can also exacerbate underlying chronic illnesses. For instance, people with asthma may experience more frequent and severe asthma attacks, while those with heart disease may have an increased risk of myocardial infarction shortly after influenza infection. Similarly, influenza has been associated with an increased incidence of stroke in susceptible individuals.
Less common but severe complications include encephalitis (inflammation of the brain), myocarditis (inflammation of the heart muscle), and myositis (inflammation of skeletal muscles), all of which may result in long-term health consequences if not managed promptly. Children, especially those under the age of 5, are also more prone to complications such as otitis media (middle ear infection), which can contribute to hearing problems.
Differences From Other Respiratory Illnesses
Influenza often presents with symptoms that overlap with other respiratory infections, making accurate diagnosis important. Unlike the common cold, which typically begins gradually and is characterized by nasal congestion, sneezing, and mild fatigue, influenza tends to have a sudden onset with high fever, severe muscle aches, intense fatigue, and headache. These features make influenza more debilitating than most colds.
Influenza also shares several features with COVID-19, such as fever, cough, fatigue, and sore throat. However, a distinguishing feature of COVID-19 is the frequent occurrence of loss of taste (anosmia) or smell (ageusia), which is uncommon in influenza. COVID-19 also tends to cause more prolonged illness, with symptoms sometimes lasting weeks, compared to the usually shorter course of influenza.
Another difference is the incubation period: influenza symptoms typically appear within 1–4 days after exposure, while COVID-19 may take 2–14 days to manifest. The transmission dynamics also differ, with COVID-19 demonstrating more frequent asymptomatic spread compared to influenza.
Because of these overlaps, diagnostic testing is crucial to confirm whether a patient has influenza, COVID-19, or another respiratory illness. Rapid influenza diagnostic tests (RIDTs), molecular tests, and PCR assays play an important role in guiding treatment decisions, such as the timely use of antiviral medications.
Causes and Transmission
Influenza is caused by influenza viruses, which belong to the Orthomyxoviridae family. These viruses specifically target the respiratory tract, infecting the nose, throat, and lungs. Infection begins when viral particles attach to and enter cells in the lining of the respiratory tract, where they replicate and spread. The rapid multiplication of the virus leads to tissue damage and inflammation, producing the characteristic symptoms of fever, cough, sore throat, and body aches.
The ability of influenza viruses to undergo frequent genetic changes, particularly antigenic drift (small mutations) and antigenic shift (major reassortments), contributes to their persistence and capacity to cause both seasonal epidemics and, occasionally, global pandemics. These changes enable the virus to evade immunity from previous infections or vaccinations, allowing new strains to spread widely.
How Influenza Spreads
Influenza is a highly contagious illness, and transmission occurs through multiple pathways. The primary route is via respiratory droplets, which are expelled when an infected person coughs, sneezes, or talks. These droplets, typically larger than 5 microns in diameter, can travel up to about 1.5–2 meters (5–6 feet) and directly enter the respiratory tract of nearby individuals.
In addition to droplets, the virus can also spread through contaminated surfaces (fomites). Influenza viruses are capable of surviving for several hours on hard surfaces such as doorknobs, phones, and countertops, and for shorter periods on softer surfaces like fabrics or tissues. When a person touches a contaminated object and then touches their eyes, nose, or mouth, the virus can enter the body and establish infection.
Transmission can also occur through aerosols, which are much smaller respiratory particles capable of remaining suspended in the air for longer periods. This mode of spread is particularly relevant in crowded or poorly ventilated indoor spaces, where viral particles can accumulate. For this reason, influenza outbreaks often spread quickly in schools, workplaces, hospitals, and public transportation.
An infected individual is generally contagious from about one day before symptoms appear until 5–7 days after illness begins. Children and people with weakened immune systems may remain infectious for longer periods. Importantly, because individuals can transmit the virus before they feel sick, influenza spreads easily and rapidly through populations.
Risk Factors
Several factors influence a person’s risk of contracting influenza or experiencing severe illness. Young children are particularly vulnerable because their immune systems are still developing and they may have limited prior exposure to influenza viruses. Adults over 65 years of age face increased risks due to weaker immune responses and the presence of other age-related health conditions.
Pregnant women are at higher risk of complications because pregnancy naturally alters the immune, heart, and lung functions, making them more susceptible to severe respiratory infections. Individuals with chronic medical conditions such as asthma, chronic obstructive pulmonary disease (COPD), diabetes, heart disease, kidney disorders, or weakened immune systems (including those undergoing cancer treatment or living with HIV/AIDS) are also more likely to develop serious complications.
Environmental and social factors further contribute to transmission risk. Influenza spreads more easily in close-contact environments such as schools, nursing homes, dormitories, and military barracks, where individuals live or work in proximity. Healthcare facilities are also high-risk settings due to frequent exposure to infected patients.
Seasonal factors play a role as well. In temperate regions, influenza activity peaks in the winter months, when people spend more time indoors and in close contact, facilitating viral spread. Cold, dry air may also help influenza viruses survive longer outside the body. In tropical regions, influenza occurs year-round, with peaks often aligning with the rainy season.
Finally, lack of vaccination significantly increases susceptibility. Annual vaccination is the most effective preventive measure, as it reduces the likelihood of infection and lowers the risk of severe illness and complications if infection does occur.
Diagnosis of Influenza
Accurate diagnosis of influenza relies on laboratory testing and clinical evaluation. Testing confirms the virus presence, while clinical criteria guide timely treatment decisions, especially during peak seasons.
Diagnostic Methods
Rapid influenza diagnostic tests detect viral antigens in respiratory specimens, such as nasal or throat swabs. They provide results within 15 to 30 minutes, making them highly useful for point-of-care decision-making in clinics and emergency settings. However, their sensitivity is relatively low—ranging from about 50% to 70%—which means false negatives are common, especially during periods of high influenza activity. Despite these limitations, RIDTs are more reliable when influenza activity in the community is elevated, as positive results are more likely to represent true infections.
RT-PCR is considered the gold standard for diagnosing influenza. This method detects and amplifies viral RNA with very high sensitivity and specificity, making it capable of confirming infections even when the amount of virus present is small. RT-PCR can also distinguish between influenza types (A and B) and specific subtypes, such as H1N1 or H3N2. While highly accurate, this test requires specialized laboratory facilities and trained personnel, with turnaround times ranging from several hours to a full day.
Immunofluorescence assays use fluorescently labeled antibodies to detect influenza antigens in respiratory secretions. These methods produce results more quickly than viral culture, but they require skilled laboratory technicians and specialized equipment, such as a fluorescence microscope. Although less sensitive than RT-PCR, DFA and IFA tests are more accurate than RIDTs, making them a useful intermediate option in some clinical and public health laboratories.
Viral culture involves growing live influenza virus from a patient’s respiratory specimen. This method takes considerably longer than other diagnostic techniques—typically three to ten days—making it unsuitable for routine diagnosis. However, it remains valuable in research and surveillance, where it is used to study virus characteristics, monitor mutations, and select strains for seasonal influenza vaccines.
Criteria for Clinical Diagnosis
In many healthcare settings—particularly where laboratory testing is unavailable or results may be delayed—clinicians rely on characteristic patterns of symptoms. Influenza often begins with the sudden onset of high fever, chills, dry cough, sore throat, muscle aches, headache, and fatigue. These symptoms appear more abruptly and are generally more severe than those associated with the common cold, helping distinguish influenza from other viral respiratory illnesses.
The likelihood of influenza increases significantly during peak flu season, which varies depending on the region. In such times, clinicians may diagnose influenza based on symptoms alone, especially if the patient has been exposed to confirmed cases or is located in an outbreak setting. This epidemiological context strengthens clinical suspicion, even in the absence of laboratory confirmation. Physical examination findings in influenza are usually nonspecific, such as fever, pharyngeal redness, and nasal congestion. While these signs can support a clinical diagnosis, they are more often used to exclude alternative conditions, such as bacterial pneumonia or streptococcal pharyngitis, rather than to definitively confirm influenza.
Certain groups of patients require special attention when assessing influenza risk. These include young children, older adults, pregnant women, and individuals with chronic health conditions such as asthma, diabetes, or heart disease. Because these populations are at higher risk of complications, clinicians may initiate antiviral treatment based on clinical suspicion alone, without waiting for confirmatory test results.
Treatment Options
Treating influenza involves targeted medications to reduce viral activity and supportive care to manage symptoms and prevent complications. Prompt intervention can lessen illness severity and duration.
Antiviral Medications
Antiviral drugs remain the cornerstone of influenza treatment, especially for vulnerable populations such as children, the elderly, pregnant women, and those with chronic medical conditions. Several classes of antivirals are available, each with different routes of administration and effectiveness profiles.
Oseltamivir, taken orally, is the most commonly prescribed neuraminidase inhibitor and is effective against influenza A and B viruses. Zanamivir, an inhaled formulation, provides similar benefits but is not suitable for patients with asthma or chronic obstructive pulmonary disease due to the risk of bronchospasm. Peramivir, given intravenously, is primarily reserved for hospitalized patients or those unable to tolerate oral medications. A newer option, baloxavir marboxil, represents a different drug class—a cap-dependent endonuclease inhibitor—and is taken as a single-dose oral therapy. It is particularly useful against strains resistant to neuraminidase inhibitors.
Antivirals are most effective when initiated within 48 hours of symptom onset, as they shorten illness duration by one to two days and significantly reduce the likelihood of severe complications such as pneumonia, hospitalization, or death. Nevertheless, in critically ill or hospitalized patients, starting antiviral therapy later can still be beneficial. For this reason, guidelines recommend administering antivirals to all patients with severe, progressive, or high-risk influenza, even when diagnosis occurs beyond the optimal treatment window.
These medications are generally well tolerated, though some adverse effects may occur. The most common are nausea, vomiting, and headache, while neuropsychiatric effects such as confusion or agitation have been reported in rare cases, particularly among children taking oseltamivir. They do not replace vaccination but serve as an essential tool in early treatment and outbreak control.
Supportive Care
Supportive care focuses on symptom relief and hydration. This form of treatment is especially important in patients with mild influenza who may not require prescription antivirals.
Symptomatic relief often includes the use of antipyretic and analgesic medications such as acetaminophen or ibuprofen to reduce fever, headaches, and body aches. For cough and nasal congestion, over-the-counter cough suppressants, throat lozenges, or humidified air may provide comfort. However, decongestants should be used cautiously in individuals with hypertension or cardiovascular disease due to potential side effects. Rest is equally important, as adequate sleep and reduced physical activity support the immune system’s response and minimize the risk of worsening illness.
Proper hydration and nutrition are also essential in recovery. Influenza often causes dehydration due to fever, sweating, or vomiting, particularly in children and the elderly. Patients are encouraged to maintain adequate fluid intake, and in severe cases, oral rehydration solutions may be required to restore electrolyte balance.
For patients with severe disease, hospitalization may be necessary. Hospital-based care can involve supplemental oxygen or non-invasive ventilation for those with respiratory distress, intravenous fluids for hydration, and, in cases of suspected bacterial coinfection, antibiotics to treat secondary pneumonia. Patients with acute respiratory distress syndrome (ARDS) may require intensive care support, including mechanical ventilation.
Certain populations require special treatment considerations. In children, aspirin must be strictly avoided because of the risk of Reye’s syndrome, a rare but potentially fatal condition. Pregnant women are at greater risk of influenza-related complications, making early antiviral therapy particularly important and safe for both mother and fetus. Elderly patients often present with frailty and multiple comorbidities, necessitating vigilant monitoring and sometimes inpatient management to prevent rapid deterioration.
Influenza Prevention
Effective prevention of influenza relies on both vaccination and consistent personal hygiene practices. These measures significantly reduce the risk of infection and limit the spread of the virus within communities.
Vaccination
Vaccination remains the most effective and widely recommended strategy for influenza prevention. The flu vaccine is reformulated each year to target the most prevalent strains predicted for the upcoming season, ensuring that protection is as relevant as possible. Because influenza viruses mutate rapidly, annual immunization is necessary to maintain adequate defense.
Health authorities such as the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) recommend vaccination for everyone aged six months and older. It is especially important for groups at higher risk of severe illness, including young children, older adults, pregnant women, and individuals with chronic diseases such as asthma, diabetes, or cardiovascular conditions. For these populations, vaccination can reduce the risk of hospitalization and life-threatening complications.
The timing of vaccination is also critical. Immunization is ideally administered before the onset of the flu season, often in early autumn, as it allows the body to develop sufficient immunity before peak transmission periods. Even in cases where vaccination does not prevent infection entirely, it has been shown to lessen the severity and duration of illness, reducing the likelihood of secondary complications like pneumonia.
In addition, herd immunity plays an important role in community protection. When a large proportion of the population is vaccinated, it lowers overall viral circulation, thereby protecting individuals who cannot be vaccinated, such as infants under six months or those with certain medical contraindications.
Personal Hygiene Measures
Personal hygiene practices help minimize the transmission of influenza.
- Regular and thorough hand washing with soap and water for at least 20 seconds is highly effective.
- When soap and water are unavailable, alcohol-based hand sanitizers (≥60% alcohol) are a reliable alternative.
- Covering the mouth and nose with a tissue or inner elbow when sneezing or coughing prevents droplet spread.
- Dispose of used tissues immediately and perform hand hygiene afterward.
- Clean and disinfect frequently touched surfaces (doorknobs, phones, keyboards, countertops) regularly.
- Avoid close contact with individuals who show flu-like symptoms.
- Sick individuals should stay home until at least 24 hours after fever subsides.
- Wearing masks in crowded or enclosed spaces reduces droplet transmission.
- Mask use is especially recommended during peak flu season or outbreaks.
- These practices protect individuals and reduce influenza circulation within the community.
Impact on Public Health
Influenza significantly affects healthcare systems and population health worldwide. Its impact is seen in recurrent seasonal outbreaks and occasional pandemics, both causing heavy burdens on medical resources and society.
Annual Outbreaks
Seasonal influenza causes millions of illnesses worldwide each year. It leads to 3 to 5 million severe cases and approximately 290,000 to 650,000 respiratory deaths globally.
Older adults, young children, and individuals with underlying conditions face higher risk of complications. Health systems mobilize vaccination campaigns annually to reduce transmission and severity.
The economic effects include increased absenteeism, medical costs, and reduced productivity. Hospitals often experience high patient loads during peak flu seasons, which strains emergency and outpatient services.
Pandemic Influenza
Pandemics occur when a new influenza virus strain emerges, easily spreading among humans due to low immunity. Notable examples include the 1918 H1N1 and 2009 H1N1 pandemics.
Pandemics result in rapid global spread with higher mortality rates compared to seasonal influenza. They demand swift public health responses including surveillance, vaccination development, and social measures.
The strain on healthcare resources intensifies due to surges in severe cases requiring hospitalization and intensive care. Public health strategies focus on containment and mitigation to reduce fatality and transmission rates.
Special Considerations
Certain groups face higher risks of serious complications from influenza and require targeted preventive measures. Awareness of these risks helps guide vaccination and treatment priorities effectively.
High-Risk Populations
Individuals with chronic health conditions such as asthma, diabetes, heart disease, and immunosuppression have increased susceptibility to severe influenza outcomes. Pregnant women also experience greater risk due to altered immune function and cardiovascular changes.
Vaccination is especially critical for these populations, as flu infections can exacerbate underlying illnesses. Antiviral medications may be administered promptly in cases of confirmed influenza to reduce complications.
Healthcare providers emphasize routine influenza vaccination annually for high-risk groups. This proactive approach reduces hospitalization and mortality rates related to influenza infections in these vulnerable patients.
Influenza in Children
Children under 5 years old, particularly those younger than 2, are at amplified risk for influenza complications, including pneumonia, dehydration, and febrile seizures. Influenza symptoms in children often present with high fever, cough, and gastrointestinal symptoms.
Prompt diagnosis and treatment with antiviral agents can reduce severity and duration. Annual immunization is recommended for all children aged 6 months and older.
Daycare and school environments contribute significantly to transmission among children. Preventive strategies include vaccination, good hand hygiene, and exclusion from group settings during illness.
Influenza in Older Adults
Adults aged 65 and above face increased morbidity and mortality from influenza due to declining immune response and the higher prevalence of comorbidities. Complications commonly include pneumonia, cardiovascular events, and functional decline.
High-dose or adjuvanted influenza vaccines are advised to improve immune response in this age group. Early antiviral treatment is critical when symptoms develop.
Regular vaccination and prompt medical attention reduce hospitalization risk and improve clinical outcomes for older adults during flu seasons.