Children have the highest infection rates for influenza viruses. Types and subtypes These rapid genetic changes are the reason that one or more new virus strains are usually incorporated into each year's influenza vaccine. A person's immunity to the surface antigens, especially hemagglutinin, reduces the likelihood of infection and the severity of disease if infection occurs. The influenza incubation period is one to four days, with an average of two days. Persons can be infectious starting the day before symptoms begin through about five days after illness onset.
Children can be infectious for a longer period.
Uncomplicated influenza is characterized by the abrupt onset of fever, myalgia, headache, severe malaise, nonproductive cough, sore throat, and rhinitis. In some people, influenza can exacerbate underlying medical conditions such as pulmonary or cardiac disease or lead to secondary bacterial pneumonia or primary influenza viral pneumonia. Although drugs are becoming available to treat influenza, the disease vaccines remain the primary method for preventing influenza and its complications. When vaccine and epidemic strains are well-matched, achieving high vaccination rates, particularly in nursing home, day care, school, and health care environments, can reduce the risk of outbreaks by inducing widespread immunity. Influenza vaccine contains two type A and one type B influenza virus strains.