The common everyday name for the illness caused by influenza viruses is the flu. Influenza is a contagious respiratory infection whose symptoms can include fever, cough, sore throat, fatigue, headache, and muscle aches. Symptoms commonly begin one to four days after exposure.
The name “influenza” came into English through Italian. In medieval and early modern Europe, influenza referred to an influence or visitation, reflecting the once-common belief that outbreaks were caused by celestial or environmental forces. The shortened word “flu” became established in English during the nineteenth century.
Flu is often confused with the common cold because both can cause congestion, coughing, and a sore throat. Influenza more often begins abruptly and causes pronounced fever, exhaustion, and body aches, although symptoms overlap and a diagnosis cannot always be made from symptoms alone. Influenza A and B viruses cause most seasonal human flu. Vaccination, hand hygiene, ventilation, and staying home while ill can reduce spread, while some people with severe or high-risk infections may receive antiviral treatment.