Although the number of persons who have been (or will be) exposed to a high concentration of HIV is quite small, these persons have a high risk of becoming infected and treatments are needed to prevent infection after such an exposure. In animal studies, AZT has prevented the development of infections after exposure of the animals to a retrovirus (the HIV is a retrovirus). In patients with AIDS, AZT has been effective in delaying the progression of the disease. For these reasons a trial of AZT is indicated.