African trypanosomes swimming in human blood
After being transmitted by the bite of a tsetse fly, African trypanosomes are able to survive extracellularly in the human bloodstream while being fully exposed to the immune system. They accomplish this feat through the expression of a series of immunologically-distinct cell surface coats. Each coat is produced from a single type of variant surface glycoprotein (VSG). Periodic switching of the single expressed VSG gene from a vast silent library enables the parasite to avoid clearance by the host's adaptive immune response, hence prolonging infection and increasing the chances of transmission.