Malaria is a protozoan disease of the red blood cells transmitted by the bite of an infected mosquito. Transmission intensity of malaria differs considerably between different areas. As malaria transmission is reduced, the possibility of epidemics increases. This is due to the fact that immunity in the population becomes low or nonexistent. The characteristics of severe malarial infections also differ according to transmission intensity. In areas of high transmission, anaemia is the most common life-threatening symptom. In areas of mesoendemic and hyperendemic transmission both anaemia and cerebral malaria are the symptoms associated with severe malaria, whilst in hypoendemic areas cerebral malaria and, in older age groups, renal failure occurs most frequently. The vectors of dengue, yellow fever and the newly minted Zika viruses differ in their ecologies from malaria vectors to such an extent that, despite feeding on the same host, they can be used as an example of ways to avoid competition.