A parasite is an organism that lives in or on another organism, called a host, and damages it. A parasitic protist called Plasmodium that is transmitted by a mosquito is responsible for a worldwide epidemic disease—malaria. Malaria occurs in tropical parts of Africa, Asia, and Latin America, and it is common in the eastern Mediterranean as well. Between 350 million and 500 million people have clinical cases of malaria, and about 1 million people die of it each year. Malaria is the leading cause of death for children younger than 5 years old in sub-Saharan Africa; somewhere on the African continent, about every 30 seconds, a child dies of malaria.

Neither the incidence of malaria nor the rate of mortality has changed very much since Plasmodium was identified as the cause of malaria nearly a century ago. The reason for the lack of progress in combating malaria is the nature of Plasmodium—it has a complex development that makes it nearly invisible to the host’s immune system (FIGURE 13-23).

Figure 13-23Malaria-infected blood cells and sickled blood cells. The malaria parasite grows inside red blood cells and destroys them. Because sickle-cell trait changes the shape of some red blood cells, these cells resist infection by malaria parasites.

The human immune system has a difficult time fighting a malarial infection because, to survive from one generation to the next, Plasmodium parasites go through a series of distinct developmental stages. And as the Plasmodium cells change from one stage to another, the parasite produces different cell surface proteins. Thus Plasmodium stays ahead of the human immune system by constantly changing the way it appears.

Although the immune system doesn’t usually have much success fighting malarial infection, some individuals produce red blood cells that are inhospitable to Plasmodium, conferring a resistance to the malaria-causing protist. These individuals carry an allele, called Hb^S, which (as we learned in Chapter 7) codes for a variant of the normal adult hemoglobin that causes hemoglobin molecules to stick together in long chains that distort the red blood cell (see Figure 13-23), giving the cell a sickled shape. The sickled red blood cells leak substances that are essential to Plasmodium, making the cells inhospitable to the parasite. Although carrying a copy of the Hb^S allele confers resistance to malaria, individuals who carry two copies of the Hb^S allele suffer from sickle-cell anemia—a severely debilitating genetic disease. Individuals with a single Hb^S allele are said to have “sickle-cell trait.”