Plasmodium, the parasite responsible for the malaria disease, is considered as a vector-borne parasite, meaning that an insect vector is responsible for its transmission. As such, this parasite is transmitted by the bite of an infected female Anopheles mosquito. Many different species of Anopheles mosquito are able to transmit the parasite responsible for malaria. However, this mosquito is not present in the entire world and prefers warmer areas. The malaria disease is then present only in a broad band around the equator corresponding to the natural habitat of the Anopheles mosquitos. Unfortunately, the habitat of the Anopheles mosquitos usually corresponds to poorer countries often lacking infrastructures to deal efficiently with the prevention and the treatment of the disease. As such, this disease is usually not present in Europe and North America (except in some southern regions where the population of Anopheles mosquitos is usually kept under control) because the mosquito responsible for its transmission is absent. It is interesting to note that, according to the Centers for Disease Control and Prevention, few cases of «airport malaria» have been reported in countries where Anopheles mosquitos are usually absent. Even though this type of event is rare, it is susceptible to occur when the infected insect is transported by aircraft and contaminate someone in a country where malaria is not present. It is nevertheless very rare, as the female Anopheles usually lives only one or two weeks and might not be able to survive enough time to bite someone in colder areas.
It is interesting to note that Plasmodium is not the only microorganism susceptible to be acquired by the bite of the female Anopheles mosquito. Actually, this insect is also able to transmit the parasites Wuchereria bancrofti and Brugia malayi responsible for the lymphatic filariasis. Furthermore, this type of mosquito can also transmit some viruses. It is interesting to note that it has been suggested that Anopheles are able to transmit an unknown virus responsible for brain tumour, as the occurrence of malaria has been associated with an increased occurrence of brain tumour.
The parasite responsible for malaria is not transmitted by contact with an infected person. However, an infected pregnant woman can transmit the parasite to its unborn child during the pregnancy or the delivery. However, the rate of transmission from a mother to the unborn child is quite low, especially if the woman is treated during the pregnancy. It is interesting to note that pregnant women are more susceptible to acquire infections in general than not pregnant women, as their immune system is less active in order to avoid provoking harmful reactions towards the foetus. Consequently, pregnant women are more susceptible to acquire the parasite responsible for malaria than other people.
It is also possible to acquire Plasmodium while receiving a blood transfusion. Although this type of situation is very rare, it can lead to severe complications. As a matter of fact, the Centers for Disease Control and Prevention estimate that there is about one case of malaria acquired by blood transfusion every two years in the United States. This type of situations is possible because there is no approved test available yet to successfully screen the donated blood for the presence of the parasite responsible of the malaria. Consequently, many countries have put measures in place to limit the presence of Plasmodium in the donated blood. These measures include questioning the potential donors in order to exclude the ones susceptible to be infected. For example, in the United States, it is not possible to donate blood for one year after traveling in a country where malaria is present. Furthermore, immigrants coming from a country where malaria is present cannot donate blood for three years after their last visit in their home country. Finally, people who had been diagnosed with malaria are authorized to give blood only three years after the end of their symptoms. Similar rules are in place in almost all countries where malaria is not present to avoid the presence of this parasite in the donated blood.
Sources: Centers for Disease Control and Prevention; Lehrer, S. (2010). Anopheles mosquito transmission of brain tumor. Medical hypotheses, 74(1), 167-168.