Influenza, commonly known as the flu, is a contagious viral infection that affects the respiratory system, including the nose, throat, and lungs. It spreads easily through droplets released when an infected person coughs, sneezes, or talks. Unlike a common cold, influenza tends to come on suddenly and can cause more severe symptoms that last for several days to weeks.

Typical symptoms include fever, chills, body aches, sore throat, fatigue, headache, and a dry cough. Some people may also experience nasal congestion or shortness of breath. In children, symptoms can include nausea or vomiting. The flu can make people feel extremely tired, even after other symptoms have improved.

Influenza viruses change frequently, which is why new strains appear each year. Seasonal flu outbreaks are most common during colder months when people spend more time indoors and in close contact. Certain groups, including young children, older adults, and those with chronic health conditions, are more at risk of severe illness or complications such as pneumonia.