…[B]y foreseeing them in a distance, which is only done by men of talents, the evils which might arise from them are soon cured; but when, from want of foresight, they are suffered to increase to such a height that theyare perceptible to everyone, there is no longer any remedy.

NICCOLO` MACHIAVELLI, 1513

Time is an important variable not only in the recognition and treatment of infection but also in the use of postexposure prophylaxis. This article highlights the use of immunologic or antimicrobial interventions to prevent or modify subsequent infection with viral hepatitis, tuberculosis, Lyme disease, several childhood diseases (measles, mumps, rubella, varicella), and two illnesses with very high mortality rates, AIDS, and rabies. Prophylaxis for meningococcal infection, Group A streptococci, diphtheria, pertussis, and tetanus are considered elsewhere in this issue of Infectious Disease Clinics of North America.