Many protozoa, such as the amoebae grazing through forest floors, for instance, or the phytoplankton that turn the oceans green, are harmless. But there are thousands of species of parasitic protozoa, and they include some of the most vicious parasites of all. By the turn of the century, scientists had figured out that the brutal fevers of malaria weren’t caused by bad air but by several species of a protozoan called Plasmodium, a parasite that lived inside mosquitoes and got into humans when the insects pierced the skin to suck blood. Tsetse flies carried trypanosomes that caused sleeping sickness. Yet, despite their power to cause disease, most protozoa couldn’t live up to Koch’s rigorous demands. They were creatures after Steenstrup’s heart, passing through alternating generations.
Plasmodium, for example, enters a human body through a mosquito bite as a zucchini-shaped form known as a sporozoite. It travels to the liver, where it invades a cell and there multiplies into forty thousand offspring, called merozoites—these are now shaped like a grape. Merozoites pour out of the liver and seek out red blood cells, where they make more merozoites. The new generations burst out of the cells and seek out more blood cells. After a while, some of the merozoites produce a different form—a sexual one, called a macrogamont. If a mosquito should take a drink of the host’s blood and swallow a blood cell with macrogamonts in it, they will mate inside the insect. The male macrogamont fertilizes the female one, and they produce a round little offspring called an ookinete. The ookinete divides in the mosquito’s gut into thousands of sporozoites, which travel to the mosquito’s salivary glands, there to be injected into some new human host.
With so many generations and so many different forms, you can’t raise Plasmodium organisms simply by throwing them in a petri dish and hoping they’ll multiply. You have to get male and female macrogamonts to believe that they’re living in the gut of a mosquito, and once they’ve bred, you have to make their offspring believe they’ve been shot out of the mosquito’s mouth and into human blood. It’s not impossible to do, but it took until the 1970s, a century after Koch set up his rules, for a scientist to figure out how to culture Plasmodium in a lab.
Parasitic eukaryotes and parasitic bacteria were pushed further apart by geography. In Europe, bacteria and viruses caused the worst diseases, such as tuberculosis and polio. In the tropics, protozoa and parasitic animals were just as bad. The scientists who studied them were generally colonial physicians, and their specialty became known as tropical medicine. Europeans came to look upon parasites as robbing them of native labor, of slowing down the building of their canals and dams, of preventing the white race from living happily at the Equator. When Napoleon took his army to Egypt, the soldiers began to complain that they were menstruating like women. Actually they had been infected with flukes. Like the flukes Steenstrup had studied, these were shed by snails and swam through water looking for human skin. They ended up in the veins in the abdomens of the soldiers and pushed their eggs into their bladders. Blood flukes attacked people from the western shores of Africa to the rivers of Japan; the slave trade even brought them to the New World, where they thrived in Brazil and the Caribbean. The disease they caused, known as bilharzia or schistosomiasis, drained the energy of hundreds of millions of people who were supposed to build European empires.
As bacteria and viruses occupied the center of medicine, parasites (in other words, everything else) were spun out to the periphery. Specialists in tropical medicine went on struggling against their own parasites, often with a staggering lack of success. Vaccines against parasites failed miserably. There were a few old cures—quinine for malaria, antimony for blood flukes—but they did only a little good. Sometimes they were so toxic that they caused as much harm as the disease itself. Meanwhile, veterinarians studied the things living inside cows and dogs and other domesticated animals. Entomologists looked at the insects dug into trees, the nematodes that sucked on their roots. All these different disciplines became known as parasitology—more of a loose federation than an actual science. If anything held together its factions, it was that parasitologists were keenly aware of their subjects as living things rather than just agents of disease, each subject with a natural history of its own—in the words of one scientist at the time, “medical zoology.”
Some actual zoologists studied this medical zoology. But just as the germ theory of disease was changing the world of medicine, they were reckoning with a revolution of their own. In 1859, Charles Darwin offered a new explanation for life. Life, he argued, hadn’t existed unchanged since Earth’s creation but had evolved from one form to another. That evolution had been driven by what he named natural selection. Every generation of a species was made up of variants, and some variants fared better than others—they could catch more food or avoid becoming food for someone else. Their descendants inherited their characteristics, and with the passing of thousands of generations, this unplanned breeding produced the diversity of life on Earth today. To Darwin, life was not a ladder rising up to the angels or a cabinet filled with shells and stuffed animals. It was a tree, bursting upward with all the diversity of the species on Earth alive today and long past, all rooted in a common ancestry.
Parasites fared as badly in the evolutionary revolution as they had in the medical one. Darwin contemplated them only in passing, usually when he was trying to argue that nature was a bad place to try to prove God’s benevolent design. “It is derogatory that the Creator of countless systems of worlds should have created each of the myriads of creeping parasites,” he once wrote. He found that parasitic wasps are a particularly good antidote to sentimental ideas about God. The way that the larvae devoured their host from the inside was so awful that Darwin once wrote of them, “I cannot persuade myself that a beneficient and omnipotent God would have designedly created the Ichneumonidae [one group of parasitic wasps] with the express intention of their feeding within the living bodies of Caterpillars.”
Yet, Darwin was downright kind to parasites compared with the later generations of biologists who carried on his work. Instead of benign neglect, or even mild disgust, they felt outright scorn for parasites. These late Victorian scientists were drawn to a peculiar, now debunked form of evolution. They accepted the concept that life evolved, but Darwin’s generation-by-generation filter of natural selection seemed too random to account for the trends they saw in the fossil record that had lasted millions of years. They saw life as having an inner force driving it toward greater and greater complexity. To their mind, this force brought a purpose to evolution: to produce the higher organisms—vertebrates such as us—from the lower beings.
One influential voice for these ideas belonged to the British zoologist Ray Lankester. Lankester grew up with evolution. When he was a boy, Darwin came to his family’s house and told him stories about riding a giant tortoise on a Pacific island. When Lankester became a man, he had a giant frame and a puffy, vaguely Charles Laughton–like face. As an Oxford professor and the director of the British Museum he carried Darwin’s theory forward with what seemed at times like sheer bodily power. He made the people around him feel small in both size and mind; he reminded one man who met him of a winged Assyrian beast. Once King Edward VII offered him some tidbit of scientific knowledge while paying him a royal visit, and Lankester bluntly replied, “Sir, the facts are not so; you have been misinformed.”