A trypanosome is a type of parasitic protozoan which can cause a number of serious diseases in people and domestic animals, including African sleeping sickness and leishmaniasis.

The trypanosome is able to remain within a host's body for a very long time because it regularly changes the proteins (i.e. antigens) on its outer surface, so that the host's immune system must constantly develop new antibodies in order to continue to recognize and destroy it. The trypanosome uses a large number of different genes which can be used in many combinations to come up with the different surface proteins. As a result, the host is never able to completely eliminate the trypanosome without the aid of antibiotics.

From the science dictionary at http://biotech.icmb.utexas.edu/