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.
The _sleeping sickness_ has many features of interest.