Flu differs in several ways from the common cold, a respiratory infection also caused by viruses. Outbreaks of flu usually begin abruptly. As the disease spreads through communities, the number of cases peaks in about 3 weeks and subsides after another 3 or 4 weeks. Besides the rapid onset of the outbreaks and the large numbers of people affected, flu is important because of the seriousness of the complications that can develop. However, for elderly people, newborn babies, and people with certain chronic illnesses, flu and its complications can be life-threatening.
Flu viruses can enter the body through the mucous membranes of the eyes, nose, or mouth.
After a person has been infected with the virus, symptoms usually appear within 2 to 4 days. The infection is considered contagious for another 3 to 4 days after symptoms appear. The greatest risk of infection is in highly populated areas, where people live in crowded conditions, and in schools. Isolating people with flu symptoms is not an effective means of disease control because flu can be spread by someone whose symptoms are not yet apparent. Typically, the fever starts declining on the second or third day of the illness.