Flu occurs worldwide, in the winter months in the northern and southern hemispheres, and sporadically year round in the tropics. In the United States, flu generally occurs from November to April. Influenza is spread by droplets generated by coughing. During epidemics, attack rates are in the 20 to 30 percent range, and may be as high as 50 percent during pandemics.1 The Centers for Disease Control and Prevention and local health departments track influenza activity monthly. Influenza A attack rates are higher for children than for adults or the elderly, but the disease carries a higher mortality rate for young adults and especially for the elderly. Influenza B produces a similar illness with high attack rates for children and lower attack rates for adults, probably because of preexisting immunity due to a lower rate of antigenic drift.

After exposure, the incubation period is usually about 2 days. Viral shedding (contagiousness) starts approximately 24 h before the onset of symptoms, rises to peak levels within 48 h, and then declines over the next 3 to 7 days. In young children. viral shedding is prolonged.

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