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.