Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness and, in some cases, can lead to death, particularly among vulnerable populations such as the elderly, young children, pregnant women, and individuals with underlying health conditions.

Influenza viruses are categorized into types A, B, and C, with influenza A and B viruses being responsible for seasonal flu outbreaks. These viruses are constantly changing through a process called antigenic drift, which is why seasonal flu vaccines are updated annually to match the circulating strains.

Symptoms of influenza typically include fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, fatigue, and occasionally vomiting or diarrhea, although these gastrointestinal symptoms are more common in children than adults.

The flu spreads mainly through respiratory droplets when an infected person coughs, sneezes, or talks. It can also spread by touching a surface or object that has the virus on it and then touching one's own mouth, nose, or possibly eyes.

Preventive measures against influenza include getting an annual flu vaccine, practicing good hand hygiene, avoiding close contact with sick individuals, covering coughs and sneezes, and staying home when sick to prevent spreading the virus to others.

Treatment for influenza typically involves rest, staying hydrated, and using over-the-counter medications to relieve symptoms. In some cases, antiviral medications may be prescribed to shorten the duration and severity of the illness, particularly for individuals at high risk of complications.