This project aims to determine whether there is any detectable impact of Koala Retroviruses on the immune system of the Koala. The Koalas at Taronga Zoo are all KoRV A positive, and some are infected with KoRV B. Researchers will take blood from the Taronga koalas and use the in vitro immunological methods of isolating immune cells from the blood and stimulating them (which replicates the situation in the body when the lymphocytes are challenged by an infection) to see if there is a difference in the immunological responses between Koalas infected with different KoRV variants and with differing KoRV loads.