Meet, if you dare, the Old World hookworm. Its flesh is a pinkish grey, and its head is slightly, creepily bent away from its body. It has a wide mouth, almost as big as the entire head, bar
Meet, if you dare, the Old World hookworm. Its flesh is a pinkish grey, and its head is slightly, creepily bent away from its body. It has a wide mouth, almost as big as the entire head, barbed with two pairs of sharp teeth. It looks like a finger with the jaws of a great white shark. And it is a parasite.
The Old World hookworm lives in the intestines of bigger animals, including many humans. It latches onto the wall of the intestines with its teeth, and sucks its host's blood. In this way, the hookworm prospers at its host's expense. Like all parasites, it takes and gives nothing back.
Surely we would be better off without such a good-for-nothing drain on resources? Suppose all the world's parasites disappeared overnight. Wouldn't all the other animals be healthier, and wouldn't there be less suffering? Surprisingly, a world without parasites might not be a nicer one. There might be just as much sickness and pain, but much of the beauty of the natural world would be obliterated. Perhaps worst of all, we might all stop having sex.
There are millions of different parasites. The word is derived from the Greek word "parásītos", which means "one who eats at the table of another". They infect and live off other species, known as hosts, benefitting at the expense of their chosen victims.
Around 50% of all organisms are parasitic
It's not just worms: many groups of organisms have some members that are parasites. Plenty of fungi are parasites, including the largest living organism on Earth, a honey fungus 2.4 miles (3.8 km) across in the Blue Mountains in Oregon. There are also parasitic plants such as mistletoe, not to mention fleas and lice, and viruses and bacteria. There are even bird parasites: cuckoos are the most famous, thanks to their cheeky habit of laying eggs in other birds' nests.
Scientists estimate that around 50% of all organisms are parasitic. "There's a huge universe of parasites," says parasite expert Andres Gomez of ICF International in Washington, DC. "They are abundant, ubiquitous, diverse and important."
Clearly, if we got rid of all the parasites, the world would look very different. You would notice the difference before the first day was out.
"Within hours, millions of poor people would be cured of serious chronic illness like malaria, schistosomiasis and ascariasis," says Kevin Lafferty of the US Geological Survey in Santa Barbara, California. "People would be able to work harder and enjoy their lives more. Their livestock and crops would be healthier too."
Our immune systems have evolved to cope with a certain amount of infections
But this honeymoon period wouldn't last long. For one thing, our bodies might rebel against it. "There could be unanticipated consequences of taking parasites away, because we've been co-evolving with them for so long," says Jaap de Roode of Emory University. "It wouldn't be good for us."
According to the "hygiene hypothesis", our immune systems have evolved to cope with a certain amount of infections. So if we aren't exposed to parasites and other diseases when we're young, our immune systems don't develop properly and can start attacking our own bodies. This may explain why so many people today, living in clean environments, suffer from allergies and autoimmune diseases. If we weren't exposed to any parasites at all, we might suffer from even more of these diseases.
What's more, it wouldn't just be our immune systems that ran out of control.
As well as harming people, parasites keep down the numbers of plant-eating insects and other animals we consider pests. Within months, these species would increase in numbers and cause serious damage to food crops, says Lafferty. As a result, we would have to use even more pesticides, which would affect wildlife. Even with the extra pesticide, "some people would start to go hungry," says Lafferty.
We'd have to kill a lot of things
"Almost every species you can think of has a parasite," says Levi Morran of Emory University in Atlanta, Georgia. "And parasites have a role in ecosystems to cull populations and keep them in check. Without parasites, populations could explode."
Without parasites, we would have to start culling species whose numbers would otherwise explode, says Morran. "That would be difficult and something humans haven't dealt with before," he says. "We'd have to kill a lot of things, so it would be rough."
And it wouldn't just be us doing the killing. There would be a cornucopia of animals and plants, that in the old world would have been killed as a result of their parasites. Something would have to eat them.
"Nature really does