d 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 a