A and B are responsible for epidemics and type C causes mild respiratory illness. The attack rate is highest in the young and mortality highest in the elderly.
New strains emerge each year as a result of antigenic variation.
The incubation period is 1-5 days. Systemic symptoms include abrupt onset of fever, chills, headaches, myalgia, and malaise. Illness typically resolves after 1-2 weeks, however fatigue may continue for several weeks past illness. Virus can be recovered from secretions for 10 days. Although uncommon, it has a high fatality rate.