Influenza
Influenza, often called flu, is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs.
What influenza is
Influenza is a contagious respiratory illness caused by influenza viruses. It mainly affects the nose, throat, and lungs. People often use the word flu casually for many illnesses, but true influenza is different from the common cold and from viral gastroenteritis, which is sometimes called stomach flu.
How it spreads
Flu viruses spread mostly through respiratory droplets and small particles released when infected people cough, sneeze, talk, or breathe. A person may also become infected after touching a contaminated surface and then touching the eyes, nose, or mouth, though this is not usually the main route. Crowded indoor spaces can make spread easier.
Symptoms
Flu symptoms often start suddenly. They can include fever or feeling feverish, chills, cough, sore throat, runny or stuffy nose, muscle aches, headache, fatigue, and sometimes vomiting or diarrhea, especially in children. Not everyone with flu has a fever, and symptoms can overlap with COVID-19, RSV, colds, and other respiratory infections.
Who is at higher risk
Some people are more likely to develop serious flu complications. Higher-risk groups include young children, adults 65 and older, pregnant people, people with asthma, COPD, heart disease, diabetes, kidney disease, weakened immune systems, some neurological conditions, and people with severe obesity. Risk also depends on access to care and timely treatment.
Testing and diagnosis
Clinicians may diagnose flu based on symptoms and local flu activity, but testing can help when results affect treatment, infection control, or care decisions. Flu tests often use a swab from the nose or throat. A negative rapid test does not always rule out flu, so clinicians interpret results with timing, symptoms, and community spread.
Treatment
Many people recover with rest, fluids, and symptom care, but flu can become serious. Antiviral medicines may reduce severity or shorten illness when used early, and they are especially important for people at higher risk of complications or those with severe disease. Antibiotics do not treat influenza itself, though they may be needed for some bacterial complications.
Prevention
Prevention includes annual flu vaccination for eligible people, staying home when sick, improving ventilation, hand hygiene, covering coughs and sneezes, masking in some higher-risk situations, and avoiding close contact with people who are ill when possible. Flu vaccines are updated because circulating influenza viruses change over time.
Why it matters
Influenza matters because it is familiar enough to be underestimated and changeable enough to remain a public-health challenge. A seasonal flu wave can strain schools, workplaces, clinics, hospitals, and families, while vaccination, early care for high-risk people, and clear communication can reduce severe outcomes.