Abstract: A treatment and prophylaxis of viral infections in mammals is provided. The method involves administering an amount of a contact-blocking viral inhibitor (CVI) sufficient to elevate the concentration of the viral inhibitor in the mammal's tissue above the naturally occurring concentration in that same tissue. Methods for purifying and concentrating CVI are presented. The cellular viral inhibitor is characterized by broad antiviral activity, potency, reversible inhibition of viral attachment, cross species activity and molecular size. Natural levels of CVI range from 6 U/0.1 ml in tears to 400 U/0.1 ml in gastric secretions, with titers between these ranges found in colostrum, milk, plasma, saliva and urine. Biochemical characterization of the CVI indicates that it resists denaturation by heat, acid and alkali, and it exhibits a molecular size on the order of 2500 daltons.