T. gondii can infect virtually all warm-blooded animals, but only cats (both wild and domestic) serve as the definitive host and can excrete up to 800 million infective oocysts in their feces (64). A recent survey of cats at spay/neuter clinics in Ohio revealed that
48% of all cats were infected with T. gondii, with a higher incidence in outdoor cats (66). These oocysts can survive for long periods in the environment and may be spread by the wind or by a variety of insects and earthworms and contaminate foods ingested by humans and other animals.

Humans are the only known host for this roundworm. Eggs passed out with feces may be ingestedby the same or another person who drinks contaminated water, eats with dirty hands, or eats uncookedvegetables that have been fertilized with contaminated human wastes.

Liver flukes have a complex life cycle involving two intermediate hosts, snails and fish.

Raw fish can contain Anisakis and some other less common parasites and, if it is to be eaten raw, should first be frozen to kill the parasites. There is a potential risk that raw shellfish will contain protozoan parasites, such as Cryptosporidium. Elderly and immunocompromised persons should avoid or be very cautious about consuming raw meat, fish, or shellfish.