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BACK IN THE BAD old Stone Age, humans had to put up with all sorts of creepy crawlies.
Parasites ran amok in people's innards, freeloading on nutrient supplies. The parasites took a toll, but over the millennia, those that killed off their meal ticket too quickly didn't make it.
The survivors of this evolutionary shakeout include parasitic roundworms and flatworms, hitchhikers that allow their human host to live on - and provide them with three square meals and a cosy home.
While this scenario might appear to be win-lose, with humans the clear losers, research now suggests that may not be the whole story. In their drive to make humans hospitable hosts, parasites have developed the ability to suppress inflammation aimed against them. And this, it turns out, isn't necessarily a bad thing.
"They have evolutionarily adapted to this long-standing interaction with their hosts - that's us - and developed strategies to help the host dampen its immune response," says Helmut Haas, an immunologist at the Research Centre Borstel in Germany.
These strategies are not subtle. But humans have survived the effects and even adapted well to them: a toned-down immunity is, perhaps, the norm.
A sober immune system might still defend against enemies while not overreacting to everyday substances in the environment, or otherwise going awry. Suddenly those prehistoric times don't sound so bad - no Crohn's disease, no multiple sclerosis, no asthma. Good old Stone Age.
In a stroke of medical inspiration as bold as it is counterintuitive, scientists are now testing this theory by treating patients with live microscopic eggs or larvae of parasitic worms designed to quell these very afflictions.
Several clinical research trials are under way and more are planned. Whether promising early results will lead to treatments for these known or suspected autoimmune conditions - and extend to allergies, type 1 diabetes, psoriasis and other cases of immune revolt - remains to be seen.
Parasitic worms, or helminths, elicit a visceral response from people - in every sense. Worms don't make for polite dinner conversation and, in industrialised countries, are considered a relic of days thankfully in the past.
The notion that helminths are bad guys has been drummed into children of Western nations for several generations, as well as their doctors. (The Latin name for one hookworm is Necator americanus, meaning American killer.) But helminths have thrived in mammals for millions of years, fine-tuning the parasite act along the way. "These parasites do some harm to the host," says immunophysiologist Derek McKay of the University of Calgary in Canada.
"But if I'm [a parasite] inside a host, at the very least I want the host to reproduce and make more host babies, since my babies will need hosts." Besides, an all-out immune war on a helminth might damage the gut. "What we have instead is détente," says David Elliott, a gastroenterologist at the University of Iowa Carver College of Medicine in Iowa City.
Only recently did the advent of better sanitation and potent drugs break up this marriage. Shoe-wearing put hookworms out of business in many Western nations. But the 20th century also ushered in asthma, allergies and a host of autoimmune diseases that had previously been uncommon, and remain so in less developed countries. All are marked by out-of-control immunity. Vanquishing worm parasites in particular might have disrupted an equilibrium that had served humans well.
Of course, parasitic worms have not really gone away. Some estimates suggest that more than one billion people, mainly in the tropics, are infected, giving scientists a chance to study the link between immunity and parasites in context.
In 2003, researchers at the University of Nottingham in England and colleagues found that toddlers living in an Ethiopian city were twice as likely to wheeze - a symptom related to asthma - than were kids living in less sanitary rural areas. Similarly, Ethiopian kids with roundworm infections were half as likely to wheeze as those free of parasitic worms.
