Malaria is one of the world's most frequent parasite infections. It is caused by a protist of the genus Plasmodium and is transmitted by mosquitos. When a mosquito carrying this protist pierces a human's skin, sporozoites are released into the host. These are the only forms of the parasite that can infect humans. The sporozoites migrate to the liver where they develop into thousands of merozoites. Each merozoite contains a single cell and so each infection is contained within a single cell. Once the patient's blood cells are infected with malaria parasites they start producing more parasites than they die. This results in a high level of infection over time. People can become infected with malaria when they are exposed to an infectious stage of the parasite (i.e., gametocytes or sporozoites). Humans can also become infected with malaria through drinking contaminated water or eating contaminated food. In addition, people can be infected with malaria if they are in contact with infected blood products or infected tissues. Finally, humans can become infected with malaria during a blood transfusion or organ transplantation. In all these cases, the parasite must reach the liver before it can infect red blood cells. In endemic areas, humans become infected with malaria frequently enough for their immune systems to build up resistance.