malaria was not one. No, I was told, you’ll want to leave it off your list. Malaria is a vector-borne disease, yes, in that insects carry it from one host to another. But vectors are not hosts; they belong to a different ecological category from, say, reservoirs; and they experience the presence of the pathogen in a different way. Transmission of malarial parasites from a mosquito to a human is not spillover. It’s something far more purposive and routine. Vectors seek hosts, because they need their resources (meaning, in most cases, their blood). Reservoirs do not seek spillover; it happens accidentally and it gains them nothing. Therefore malaria is not zoonotic, because the four kinds of malarial parasite that infect humans infect only humans. Monkeys have their own various kinds of malaria. Birds have their own. Human malaria is exclusively human. So I was told, and it seemed to be true at the time.
The four kinds of malaria to which these statements applied are caused by protists of the species Plasmodium vivax, Plasmodium falciparum, Plasmodium ovale, and Plasmodium malariae, all of them belonging to the same diverse genus, Plasmodium, which encompasses about two hundred species. Most of the others infect birds, reptiles, or nonhuman mammals. The four known for targeting humans are transmitted from person to person by Anopheles mosquitoes. These four parasites possess wondrously complicated life histories, encompassing multiple metamorphoses and different forms in series: an asexual stage known as the sporozoite, which enters the human skin during a mosquito bite and migrates to the human liver; another asexual stage known as the merozoite, which emerges from the liver and reproduces in red blood cells; a stage known as the trophozoite, feeding and growing inside the blood cells, each of which fattens as a schizont and then bursts, releasing more merozoites to further multiply in the blood, and causing a spike of fever; a sexual stage known as the gametocyte, differentiated into male and female versions, which emerge from a later round of infected red blood cells, enter the bloodstream en masse, and are taken up within a blood meal by the next mosquito; a fertilized sexual stage known as the ookinete, which lodges in the gut lining of the mosquito, each ookinete ripening into a sort of egg sac filled with sporozoites; and then come the sporozoites again, bursting out of the egg sac and migrating to the mosquito’s salivary glands, where they lurk, ready to surge down the mosquito’s proboscis into another host. If you’ve followed all that, at a quick reading, you have a future in biology.
This elaborate concatenation of life-forms and sequential strategies is highly adaptive and, so far as mosquitoes and hosts are concerned, difficult to resist. It shows evolution’s power, over great lengths of time, to produce structures, tactics, and transformations of majestic intricacy. Alternatively, anyone who favors Intelligent Design in lieu of evolution might pause to wonder why God devoted so much of His intelligence to designing malarial parasites.
Plasmodium falciparum is the worst of the four in terms of its impact on human health, accounting for roughly 85 percent of reported malaria cases around the world—and for an even larger proportion of the fatalities. This form of the disease, known as falciparum malaria or malignant malaria, kills more than a half million people annually, most of them children in sub-Saharan Africa. Some scientists have suggested that the high virulence of P. falciparum reflects the fact that it’s relatively new to humans, having shifted to us within the recent past from another animal host. That suggestion has led researchers to investigate its ancestral history.
Of course, everything comes from somewhere, and because we humans ourselves are a relatively new primate, it was always logical to assume that our oldest infectious diseases had come to us—transmogrified at least slightly by evolution—from other animal hosts. It was always sensible to recognize that the distinction between zoonotic diseases and nonzoonotic diseases is slightly artificial, involving a dimension of time. By a strict definition, zoonotic pathogens (accounting for about 60 percent of our infectious diseases, as I’ve mentioned) are those that presently and repeatedly pass between humans and other animals, whereas the other group of infections (40 percent, including smallpox, cholera, measles, and polio) are caused by pathogens descended from forms that must have made the leap to human ancestors sometime in the past. It might be going too far to say that all our diseases are ultimately zoonotic, but zoonoses do stand as evidence of the infernal,