After one hundred fifty years of experiments, parasitologists can show just how extraordinary they are. Consider the blood fluke Schistosoma mansoni, a tiny missile just emerged from its snail and swimming through a pond in search of a human ankle. If it feels the ultraviolet rays of the sun, it stops swimming and sinks back down into the darkness to hide from the damaging radiation. But if it senses molecules from human skin, it begins to swim madly, jerking around in different directions. When it reaches the skin, it drills its way in. Human skin is far tougher than the soft flesh of a snail, so the fluke lets its long tail snap off, the wound quickly healing as it burrows in. Special chemicals it releases from its coat soften up the skin, letting it plunge into its host like a worm in mud.

After a few hours it has reached a capillary. It has traded the streams of the outside world for the internal ones. These capillaries are barely wider than the fluke itself, so the fluke needs to use a pair of suckers to inch forward. It makes its way to a larger vein, and a larger one still, finally making its way into a torrent of blood so powerful it carries the fluke away. The parasite rides the surge until it finally reaches the lungs. It moves from the veins to the arteries like a snake in a forest canopy. Finding its way back into a lung capillary, and then to a major artery, it is swept through the body once again. It may tour its host’s entire body three times until it finally comes to rest in the liver.

Here the fluke lodges itself in a vessel and finally has its first meal since leaving the snail: a drop of blood. It now begins to mature. If it’s a female, a uterus starts to take shape. If it’s a male, eight testes form like a bunch of grapes. In either case, the fluke grows dozens of times bigger in a few weeks. Now it is time for the parasite to search for a partner for life. If it is lucky, other flukes sniffed out this human host and are lodged in the liver as well. The females are delicate and slender; the males are shaped something like a canoe. They begin to make blood-borne odors that lure members of the opposite sex, and once a female encounters a male, she slips into his spiny trough. There she locks in, and the male carries her out of the liver. Over the course of a couple of weeks, the pair make the long journey from the liver to the veins that fan out across the gut. As they travel the male passes molecules into the female’s body that tell her genes to make her sexually mature. They keep traveling until they reach a resting place unique to their own species. Schistosoma mansoni stops near the large intestine. If we were following Schistosoma haemotobium, it would take another route to the bladder. If we were following Schistosoma nasale, a blood fluke of cows, it would take yet another route to the nose.

Once they find their destined place, the fluke couple stay there for the rest of their lives. The male drinks blood with his powerful throat and massages the female to help thousands of blood cells flow into her mouth and through her gut; he consumes his own weight in glucose every five hours and passes on most of it to her. They may be the most monogamous couples in the animal kingdom—a male will clasp onto its female even after she has died. (A few homosexual flukes will also get together. While their fit isn’t as tight, they will keep reuniting if a disapproving scientist should separate them.)

Heterosexual flukes mate every day of their long lives, and whenever the female is ready to lay her eggs, the male makes his way along the wall of the bowels until he finds a good spot. The female slides partially out of the trough, far enough to lay her eggs in the smallest capillaries. Some of the eggs are carried away by the bloodstream and end up back in the liver, that meaty filter, where they lodge and inflame the tissue, causing much of the agony of schistosomiasis. But the rest of the eggs work their way into the intestines and escape their host, ready to slice open their shells and find a new snail.

(In song):

Surfin' the waves

It's what I do

In my little

Blood canoe