1.  The insect may play a purely passive part in that its exterior surface becomes contaminated with the discharges of the sick person, and in this way the organisms of disease may be conveyed to articles of food, etc.  The ordinary house fly conveys in this way the organisms of typhoid and dysentery.  Flies seek the discharges not only for food, but for the purpose of depositing their eggs, and the hairy and irregular surface of their feet facilitates contamination and conveyance.  When flies eat such discharges the organisms may pass through the alimentary canal unchanged and be deposited with their feces; they also often vomit or regurgitate food, and in this way also contaminate objects.  Flies very greedily devour the sputum of tuberculous patients, and the tubercle bacilli contained in this pass through them unchanged and are deposited in their feces.

[Illustration:  FIG. 19.—­TRYPANOSOMES FROM BIRDS.  All the trypanosomes are very much alike.  They contain a nucleus represented by the dark area in the centre and a fur-like membrane terminating in a long whip-like flagellum.  They have the power of very active motion within the blood.]

2.  Diseases which are localized in the blood are transmitted by biting flies.  The biting apparatus becomes contaminated with the organisms contained in the blood, and these are directly inoculated into the blood of the next victim.  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.  In certain regions of Africa where a biting fly, the Glossina morsitans, occurs in large numbers, it has long been known that cattle bitten by these flies sickened and died, and this prevented the settling and use of the land.  In the blood of the sick cattle swarms of trypanosomes are found.  The source from which the tsetse fly obtained the trypanosomes which it conveyed to the cattle was unknown until it was discovered that similar trypanosomes exist in the blood of the wild animals which inhabit the region, but these have acquired by long residence in the region immunity or adaptation to the parasite and no disease is produced.  With the gradual extension of settlement of the country and the accompanying destruction of wild life the disease is diminishing.  Some of the inter-relations of infections are interesting.  The destruction of wild animals in South Africa which, by removing the sources of nagana, rendered the settlement of the country possible was due chiefly to the introduction of another infectious disease, rinderpest, which not only destroyed the wild animals but produced great destruction of the domestic cattle as well.