The number of preschool-aged children who attend day care has increased dramatically in recent years. Factors promoting spread of infections in this setting include crowding, lack of hygiene, high prevalence of early exploratory behaviors, and the likelihood of many susceptible children being in close contact. As a result, children attending day care experience a great number of episodes of respiratory and gastrointestinal illness than do other children. Moreover, the risk of a number of specific infections, including Haemophilus influenzae type b and hepatitis A, is increased by attendance in day care. Day-care staff are at increased risk of a number of infections, some of which, including cytomegalovirus and parvovirus B19, may have adverse consequences to a fetus. The presence of children in day care increases the risk of illness among staff and family members and may promote the circulation of infections in the community as a whole.