Cytauxzoon felis is a protozoal disease affecting domestic cats primarily seen in the south-central United States. The disease is tick-transmitted and, until recently, reported to be nearly 100% fatal. Once a tick has given the disease to a cat, there is rapid progression and death occurs within a week, with or without treatment. Recently, cases of domestic cats in a certain geographic location in western Arkansas and eastern Oklahoma have survived the disease and appear to have developed immunity. It is thought these cats are infected with a less aggressive strain of Cytauxzoon felis. Currently, there is no way to differentiate the different strains of the organism, and these researchers will develop a test.