Zoonotic diseases are infectious diseases transmitted to humans by vertebrate animals, such as rabies from the bite of an infected dog, bat, skunk, etc.

Insects transmit an array of so-called "vector-borne" illnesses, such as malaria, Lyme disease, and West Nile disease.

In addition, there are invertebrate worms (helminths) that are infectious parasites. They are multicellular and have differentiated tissues. Because they are animals, their physiology is similar in some ways to ours. This makes parasitic helminth infections difficult to treat because drugs that kill helminths are frequently very toxic to human cells. Many helminths have complex reproductive cycles that include multiple stages, many or all of which require a host. Examples include, schistosomiasis, which is endemic in Africa and Latin America, and trichinosis, caused by the roundworm Trichinella spiralis. This infectious agent is typically ingested in improperly cooked pork from infected pigs.