Coloured scanning electron micrograph (SEM) of Leishmania mexicana protozoans (yellow) attacking a macrophage white blood cell. This parasitic protozoan causes the tropical disease leishmaniasis. It is transmitted by the bites of infected sandflies, causing a skin ulcer at the site of the bite. The macrophage engulfs (phagocytoses) the protozoans to destroy them. However, L. mexicana is able to survive the attack and reproduces in the macrophage. When another sandfly bites the host it becomes infected, starting the cycle again. Treatment is with anti-protozoan drugs.