The genus Trypanosoma includes parasites responsible for important human diseases like sleeping sickness (Trypanosoma brucei) and Chagas disease (Trypanosoma cruzi). One of the traits that makes these parasites particularly hard to combat is their incredible ability to evade the immune system, facilitated largely by the presence of a Variable Surface Glycoprotein coat, or VSG coat. This is a dense layer of proteins that coats the outside of the parasite's cell and provides dual protection: first, it prevents host immune molecules from accessing the membrane and other associated proteins. Second, it is periodically changed for new VSG proteins that are very antigenically different, allowing the parasite to "hide" from the immune system. The trypanosome genome contains thousands of distinct VSG genes, but only one is expressed at a time.