Rabies is an acute viral infection is transmitted to humans by a bite or by the exposure of broken skin to an infected animal's saliva. Immunization given early (preferably within 24 hours but certainly within 72 hours) can usually prevent the disease. Update Date: 09/28/2008Jatin M. Vyas, PHD, MD, Instructor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.