nd
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. In the old
slavery days it was found that the negroes from the Congo region in
the course of the voyage or after they were landed sometimes were
affected with a peculiar disease. They were lethargic, took little
notice of their surroundings, slept easily and finally passed into a
condition of somnolence in which they took no food and gradually died.
There was no extension of the disease and it was attributed to extreme
homesickness and depression. A similar disease has been known for more
than one hundred years on the west coast of Africa, and attracted a
good deal of interest and curiosity on account of the peculiar
lethargy which it produced and from which it has received the name of
"sleeping sickness." Although apparently infectious in its native
haunts, it lost the power of spreading from man upon removal to
regions where it did not prevail. At first confined to a very small
region on the Niger river, it gradually extended with the development
of trade routes and the general increase of communications which trade
brings, until it prevails in the entire Congo basin, in the British
and German possessions in East Africa, and is extending north and
south of these regions. The cause of the disease and its mode of
conveyance was discovered in 1903. The fly _Glossina palpalis_
which conveys the disease is a biting fly about the size of the common
house fly and lives chiefly in the vicinity of water. When such a fly
bites an individual who has sleeping sickness its bite can convey the
disease to monkeys, on whom the transmission experiments were made.
After biting the fly is infectious for a period of two days. After
this it is harmless, unless it again obtains a supply of living
trypanosomes. There is quite a period in which there are no symptoms
of the disease, although trypanosomes are found in the blood and in
the lymph nodes, and the individual is a source of infection. The
peculiar lethargy which has given the disea