The flu — short for influenza — is an illness caused by influenza viruses. The flu spreads from person to person through the air by coughing, sneezing or just breathing. Symptoms usually appear suddenly and include fever, sore throat, cough, headache, chills and muscle aches. Most people who get the flu will recover in a week or two, but some people have more serious complications such as ear and sinus infections, pneumonia, bronchitis and inflammation of the heart.
Even healthy people can get the flu, and it can be very serious. The flu causes more deaths each year than any other vaccine-preventable disease — mostly among older people, but it is also very dangerous for children and young adults.
CDC – Flu Symptoms & Diagnosis