Much of the data presented in this review was obtained from work published during the 1960's or before. In addition, a great number of the publications describe studies conducted with only a few subjects. Because of the importance of the topic it seems appropriate that more studies with greater numbers of subjects and more up-to-date techniques should be performed. These studies should include not only respiratory and enteric viruses, but also viruses that enter the body through the skin and other routes. With the exception of respiratory syncytial virus, the infectivities of viruses that replicate in the respiratory tract appear to be comparable in man and in tissue culture. The few studies conducted with enteric viruses suggest that a low number of tissue culture infectious doses may also infect highly susceptible subjects, such as infants, but susceptible healthy adults may require much higher viral doses for infection than cultured cells.