flu

Influenza, commonly called the flu, is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. In humans, influenza A and B cause seasonal epidemics; influenza C generally causes milder illness, and influenza D primarily affects cattle. The viruses spread mainly through respiratory droplets and, to a lesser extent, by contact with contaminated surfaces.

Symptoms usually begin abruptly and include fever or chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, fatigue, and sometimes nausea or diarrhea, especially in children. Some people experience dizziness or shortness of breath. Illness can range from mild to severe, and complications can occur, particularly for older adults, pregnant people, young children, and those with certain chronic conditions.

Diagnosis is often clinical, but rapid influenza diagnostic tests detect viral antigens with variable sensitivity. Molecular tests such as RT-PCR are more accurate and can distinguish influenza from other respiratory viruses.

Treatment includes supportive care (fluids, rest, analgesics) and antiviral medications such as oseltamivir, zanamivir, or baloxavir, especially when started within 48 hours of symptom onset. Antivirals are recommended for high‑risk individuals and hospitalized patients. Antibiotics are not effective against influenza itself but may be used for suspected bacterial coinfection.

Prevention centers on annual vaccination, which is updated to match circulating strains and available in several formulations. Effectiveness varies by season and age. Additional measures include hand hygiene, respiratory etiquette, avoiding close contact with sick people, and staying home when ill.

Epidemiology: Seasonal influenza affects millions worldwide each year and causes substantial illness and death. Pandemics occur when novel influenza A viruses emerge; historically notable events include pandemics in 1918, 1957, 1968, and 2009. Public health surveillance informs vaccine strain selection and antiviral use.