What Are Parasites? Life at Someone Else's Expense

Parasites are grouped by where they live and what they are:

• •Ectoparasites live on the outside: lice, fleas, ticks, mites and the lampreys and vampire bats that feed on blood and leave
• •Endoparasites live inside: tapeworms and roundworms in the gut, flukes in the liver and blood, and the single-celled protozoa such as the malaria parasite in red blood cells and Toxoplasma in the brain
• •Parasitic plants: mistletoe, which taps a tree's sap, and the rafflesia of Borneo, which has no leaves or stem at all and exists inside a vine until it bursts out as the world's largest flower
• •Parasitoids: wasps and flies whose larvae eat a host from inside and do kill it, which is the exception to the rule and the basis of biological pest control
• •Brood parasites: the cuckoo and the cowbird, which lay their eggs in other birds' nests and let strangers raise their young

Many parasites need more than one host, and the passage between them is the hardest part of their lives. The liver fluke of sheep spends a stage in a pond snail, leaves it as a swimming larva, encysts on grass and waits to be eaten. The tapeworm that infects humans lives its adult life in the gut and its larval life in the muscle of a pig or a cow, and moves between them in food and faeces. Malaria alternates between mosquito and human, changing form half a dozen times along the way, and the complexity of the cycle is a defence, since any stage interrupted breaks the chain, which is what mosquito nets do. The cost of that complexity is that most parasite offspring die without finding the next host, and the answer is numbers: a female roundworm lays two hundred thousand eggs a day.

The most remarkable parasites do not wait for chance to move them on. A flatworm that must get from a snail into a bird makes the snail's eyestalks swell and pulse with colour so that birds peck them off. A hairworm that grows inside a cricket needs to reach water to breed, and it drives the cricket to jump into a pond and drown. Toxoplasma gondii, which reproduces only in cats, removes a rat's fear of cat urine so that the rat is eaten; about a third of humans carry it, and studies linking it to changes in personality and reaction time are suggestive and unresolved. The fungus Ophiocordyceps grows through an ant, marches it up a plant stem, makes it clamp its jaws to a leaf vein at a precise height and humidity, and then sprouts from its head to rain spores on the colony below. In each case a parasite with no brain is steering an animal that has one.

Humans are host to some three hundred species of parasitic worm and seventy of protozoa, and the diseases they cause are the largest burden of illness that rich countries have forgotten. Malaria kills around 600,000 people a year, mostly children in Africa; soil-transmitted worms infect over a billion people and stunt the growth of children; schistosomiasis, from flukes in freshwater snails, affects 200 million; and the sleeping sickness, Chagas disease, leishmaniasis and river blindness of the tropics are on the list of neglected diseases precisely because their sufferers are poor. The Guinea worm, a metre-long parasite that emerges through the skin of the leg, has been reduced from three and a half million cases a year in 1986 to a handful by nothing more than filtering drinking water, and will be the second human disease ever eradicated. Meanwhile the hygiene hypothesis proposes that the rise of allergy and autoimmune disease in rich countries reflects an immune system that evolved to fight worms and now has nothing to do, and trials of deliberate hookworm infection for Crohn's disease and asthma have been run, so far without clear success.

Parasites were ignored by ecologists for a century because they were small and hidden, and they turn out to run much of what happens. In a Californian estuary the biomass of parasites in the mud was found to exceed that of the birds. They control populations, regulate competition between species by handicapping the winners, drive the evolution of sex itself, on the theory that recombining genes each generation is the only way to keep ahead of fast-evolving parasites, and shape the bright plumage and elaborate courtship of animals, which advertise a body free of infection. A world without parasites would not be healthier; it would be one in which the commonest kind of life on Earth had been removed, and the free-living remainder would not know what to do.

The idea that a dead host is a lost home suggests that every parasite should evolve towards harmlessness, and for much of the twentieth century biologists assumed exactly that. The best-known test came when the myxoma virus was released in Australia in 1950 to control rabbits. At first it killed almost every rabbit it infected, but within a few years milder strains dominated in the wild, while the rabbits themselves evolved some resistance. A virus that killed its host within days had less time to be carried onward by mosquitoes and fleas than one that left the rabbit alive and infectious for weeks.

The same case also showed that harmlessness is not the endpoint. The virus did not become benign; it settled at an intermediate level, because the damage a parasite does is often tied to how many copies of itself it makes, and more copies can mean more chances to spread. Biologists call this the virulence trade-off. It predicts, broadly, that parasites carried by insects or water can afford to incapacitate their hosts, since a bedridden host still passes them on, while those that rely on a host walking around and meeting others tend to be milder. The common cold virus fits the second pattern.

Parasites are often described as degenerate, because many have lost organs their free-living relatives keep. An adult tapeworm has no mouth or gut at all; it absorbs digested food directly through its outer surface, which is folded into tiny projections that increase the area in contact with your intestine. Losing a gut is not a failure of evolution but a sensible saving, since the host has already done the digesting. The tapeworm's body is mostly devoted to making eggs, which is the part of its life that really matters.

Where parasites have become simpler in one way, they are often extraordinarily sophisticated in another, especially in dodging the immune system. The trypanosome that causes sleeping sickness is covered in a coat of a single protein, and as the host's antibodies learn to recognise it, a few parasites switch to a different coat from a large library of genes, so the infection returns in waves. Other parasites hide inside host cells, disguise themselves with host molecules, or release chemicals that calm the immune response. These tricks are a major reason vaccines against parasites have been so difficult to make.

Parasites leave traces that last for thousands of years, and a field called palaeoparasitology reads them. The tough shells of worm eggs survive in mummies, fossilised dung and the soil of old latrines, and under a microscope they can often be identified to species. Ötzi, the man found in the Alpine ice who died over five thousand years ago, carried whipworm. In the early twentieth century Marc Armand Ruffer found calcified schistosome eggs in the kidneys of Egyptian mummies, showing that the disease is ancient along the Nile. Soil taken from the pelvis of Richard III's skeleton, found in Leicester in 2012, held roundworm eggs.

Lice tell an even stranger story. Human body lice live in clothing and visit the skin only to feed, and their genes show that they branched off from head lice, which live in hair. Because that split could only happen once people wore clothes regularly, measuring how far the two lineages have drifted apart gives a date for the invention of clothing, which no fossil records directly; one genetic study put it somewhere between about 80,000 and 170,000 years ago. Human pubic lice are a separate case: their closest relatives live on gorillas, suggesting that our ancestors picked them up from gorillas millions of years ago.

A parasite is an organism that lives on or in a host, feeds on it and usually keeps it alive; the group includes lice and ticks on the outside, worms and protozoa inside, parasitic plants, wasps that consume their hosts, and cuckoos, and many move through complex life cycles with several hosts, some by steering their hosts' behaviour. They cause the largest neglected burden of human disease, and in ecology they outweigh top predators, regulate populations and may be the reason sex exists.