Malaria is caused by a parasite that lives in blood and is transmitted between people through mosquito bites. The sexual form of the parasite, Plasmodium falciparum, goes through some stages of its development in mosquitoes, but spends one very important stage in human red blood cells found in bone marrow. Once in the blood, these cells give the impression of being healthy because they are squishy, which allowed them to slip by the spleen, which normally looks for abnormal or dead blood cells, which are firmer, and filters them out.