At least 12 cases of severe or fatal roundworm encephalitis have been identified since 1981 in the United States (California, Illinois, Michigan, Minnesota, New York, Oregon, and Pennsylvania). Ten of the 12 cases occurred in children.

Baylisascaris procyonis (BP) is a roundworm found in the small intestine of raccoons. The highest prevalence of infection is in raccons (68%-82%) in the Midwest, Northeast, and on the West coast. Infected raccons commonly shed millions of BP eggs daily in their feces. The eggs are resistant to most environmental conditions and with adequate moisture can survive for years.

Humans become infected by ingesting infectives eggs; from the gastrointestinal tract, the larvae migrate to various tissues, viscera, eyes, and the central nervous system.