The Triumph of Parasites

Wherever there is life, there are parasites. There are 10 billion viruses in every quart of seawater. There are parasitic flatworms that can live in the bladders of desert toads, which stay buried underground 11 months of the year; there are parasitic crustaceans that live only in the eye of the Greenland shark, which swims the icy darkness of the Arctic Ocean.

As much as we’d like to ignore parasites, they are among evolution’s great success stories. They have probably existed in one form or another for billions of years. Biologists even suspect that certain viruses made out of RNA are actually survivors from the RNA world that predated DNA‑based life. Judging from their abundance today, parasites have had a happy reign on Earth. In addition to viruses, many lineages of bacteria, protozoa, fungi, algae, plants, and animals have taken the parasitic path. By some estimates, four out of every five species are parasites.

Parasites and their host are fundamentally no different from beetles trying to devour the leaves of a tree. The parasites must consume their host to survive, and their host must in turn defend itself. These twin imperatives create a fierce coevolutionary struggle. Any adaptations that can keep a host disease‑free will be favored by natural selection. Leaf‑rolling caterpillars, for instance, fire their droppings out of an anal cannon, so that they don’t end up creating a fragrant pile of frass that attracts parasitic wasps. Chimpanzees seek out foul‑tasting, parasite‑killing plants when they get intestinal worms. And when faced with an unbeatable parasite, some hosts try to make the best of a bad situation. When male fruit flies in the Sonora Desert are attacked by bloodsucking mites, they go into a mating frenzy in order to pass on as many of their genes as possible before they die.

Parasites in turn have evolved ways around their host’s defenses. Once a parasite gets inside its host, it has to escape the assault of immune cells, which blast it with poisons, suffocate it by plugging up its membrane channels, or just swallow it whole. Parasitic invaders use camouflage and subterfuge to survive. They may carry surface proteins that are exquisite forgeries of the proteins that our own bodies produce. Some of them use their mimicry to slip into cells through guarded passageways. Some parasites can jam the communication systems used by the immune system to spread the news of infection. Some can even send signals of their own that force immune cells to commit suicide. But as parasites evolve these ways around immune systems, hosts evolve new ways to kill the parasites, and the race rolls onward.