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 disease its name does not appear until the nervous system is invaded by the parasites.  It is impossible to compute accurately the numbers of deaths from this disease—­in the region of Victoria Nyanza alone the estimates extend to hundreds of thousands.

3.  In the third mode of insect conveyance the insect does not play a merely passive role, but becomes a part of the disease, itself undergoing infection, and a period in the life cycle of the organism takes place within it.  In all these cases quite a period of time must elapse before the insect is capable of transmitting the disease; in malaria, which is the best type of such a disease, this period is ten days.  Malaria is due to a small protozoan, the Plasmodium malariae, which was discovered by Lavaran, a French investigator, in 1882.  The organism lives within or on the surface of the red blood corpuscles.  It first appears as a very minute colorless body with active amoeboid movements, and increases in size, attacks