The trypanosome diseases form the best example of this mode of transmission.

The trypanosomes are widely distributed, exclusively parasitic, flagellated protozoa which live in the blood of a large number of animals and birds (Fig.

19).

They may give rise to fatal diseases, but in most cases there is mutual adaptation of host and parasite and they seem to do no harm.

One of the most dangerous diseases in man, the African sleeping sickness, is caused by a trypanosome, and the disease of domestic cattle in Africa, nagana, or tsetse fly disease, is also so produced.