The genome of P. falciparum contains between 50 and 60 var genes that code for different antigens, all of which can lead to malaria. Despite this diversity, only a relatively small number of strains of malaria become established in any population: moreover, these strains tend to have little or no genetic overlap with each other. (A) This illustration (created by Artzy-Randrup et al. and motivated by real data) shows how 40 var genes (circles) can lead to 8 different strains of P. falciparum malaria (each composed of a subset of 6 genes). Five of these strains are unique as they do not genetically overlap with any of the other strains; two strains (shown in red and light blue) share all of their genes with other strains, and one strain (shown in purple) shares four of its genes with other strains. (B) The number of hosts immune to two strains of malaria versus age: strain 1 (orange) is genetically unique, so immunity to this strain is gained only through exposure to it; strain 2 (red) shares genes with other strains of malaria, so immunity to this strain can be acquired through exposure to it and also through exposure to two other strains of malaria. Immunity to strain 1 is higher than for strain 2 as human hosts are exposed to strain 1 at a higher level and at younger ages than strain 2. The grey bars show the host age distribution.
FIGURE: ARTZY-RANDRUP ET AL.