Trypanosome

Trypanosomes are tiny single cell parasites that live in your blood and are transmitted by biting flies. They are able to survive in our body because they keep changing their surface coat so the body's defence system keeps forgetting they are there! They are able to swim through your blood using their flagellum - a whip-like structure which beats pulling the cell along. These parasite are only found in Africa so you're safe in Oxford.