Baylisascaris procyonis is a large nematode of the order Ascaridida, specific for raccoon (Procyon lotor). In North America, raccoons are extremely common in rural, suburban, and urban settings, where they have become well adapted to living alongside people. In the 1930s raccoons were introduced into Europe (i. a. Poland) and Asia for the commercial fur trade and into Japan as pets. The prevalence of B. procyonis infection in raccoons is often high, and infected animals can disseminate in their feces enormous numbers of parasite eggs. Raccoons defecate in preferred communal sites, termed latrines which play a vital role in the transmission dynamics of B. procyonis. Intestinal infections of non-raccoon species have been documented in dogs, rabbits in Japan and experimentally in opossums. Over 100 species mammals and birds can be paratenic host for B. procyonis. This parasite has emerged in recent years as one of the most serious causes of zoonotic visceral, ocular, and neural larva migrans and, in particular, of devastating encephalitis in young children. Several probable or confirmed cases of severe or fatal human B. procyonis infection have been documented. Diagnosis of Baylisascaris encephalitis is based on clinical central nervous system disease, peripheral and cerebrospinal fluid eosinophilia, deep white matter lesions visible by magnetic resonance imaging, and positive results of serologic tests. Treatment efficacy in clinical cases is poor, but albendazole prevents disease if given promptly after infection. While human baylisascariasis appears to be rare, the devastating neurologic disease that is caused by this infection and the lack of effective treatment make it a disease of public health importance. Certain characteristics of B. procyonis make it a feasible bioterrorist agent, because eggs can survive in the environment for extended periods of time, and the infectious dose of B. procyonis is relatively low. Moreover, the organism causes a severe, frequently fatal infection in humans, and no effective therapy or vaccine exists.