Astroviruses are endemic throughout the world, and infections occur year-round but with a winter peak (like rotaviruses).  Many infections are subclinical, but 4-8% of infantile gastroenteritis is attributable to astroviruses.  Epidemics are most likely to occur in the community, in day-care centers and in kindergartens, and nosocomially in pediatric wards.  Most other children and adults are immune, but outbreaks have been recorded among immunosuppressed and geriatric patients in hospitals and nursing homes.