Every year, widespread outbreaks of respiratory illness caused by influenza occur during late fall or early winter. The illness occurs throughout the world. Although many respiratory viruses can cause the symptoms of influenza, the influenza A or influenza B virus is usually responsible for the epidemics in the late fall or winter. The virus is spread by inhaling infected droplets that have been coughed or sneezed out by an infected person or by having direct contact with an infected person's secretions. Handling infected household articles may sometimes be responsible.
Influenza differs from the common cold.
Symptoms start 24 to 48 hours after infection and can begin suddenly. Chills or a chilly sensation may be the initial indication of influenza. Headache is often severe, with aching around and behind the eyes. Bright light may make the headache worse. At first, the respiratory symptoms may be relatively mild, with a scratchy sore throat, a burning sensation in the chest, a dry cough, and a runny nose.