Consider tapeworms, which attach to intestinal walls and absorb their food from the food you ate, growing to twenty or thirty or forty feet long. Or liver flukes, which squirm through your blood vessels until they eventually get to your liver, where they produce thousands more like themselves, causing trouble and pain. Or the tropical guinea worms, which have to be slowly cranked out of your legs by winding them on a stick, day after day. Or the little one-celled organisms that encyst in muscle cells or on the surface of your brain, waiting for a chance to break out. Just thinking about animal parasites is enough to give you the creeps!
Parasites of one kind or another have been with us forever, picked up from the food we eat and the water we drink, from domestic animals and the soil. If we add in microparasites, such as bacteria and viruses, they are in the air as well.
That’s why we have the complex set of defensive mechanisms known as the immune system. Some of its tricks are inherent and ever-present; others can be induced to action by parasitic invasions.
We, and almost all other multicellular organisms (and some unicellular ones, too) have another, more long-term defense mechanism, namely sexual reproduction. This is a rather effective way of regularly mixing up genes between generations, such that offspring are not genetically identical to their parents. If the genes were not rearranged and recombined, it would be easy for parasites to adapt to their genetically identical hosts, perhaps with devastating effect. Parasites can reproduce rapidly and often have high mutation rates; they commonly can change their ‘coats’, so to speak, appearing in different guises to the invaded host. So they can quickly discover new ways of finding, invading, and occupying their hosts. If all the hosts had the same defenses, it wouldn’t be long before some mutant parasite found a way to breach the defenses.
In effect, there is a continual arms race between parasite and host. In addition to their other advantages, as just mentioned, animal parasites generally also have sexual reproduction, providing yet another means of changing rapidly. As the parasites rapidly evolve new ways to exploit their hosts, the hosts—somewhat more slowly, perhaps—evolve new defenses aided by genetic recombination each generation.
Some time, many millions of years ago, a unicellular organism invented sex. In doing so, it escaped some of the damaging invasions of bacteria and viruses, and left more offspring for the next generations than its less inventive brethren. Later on, all the multicellular creatures evolved, providing new potential hosts for both microparasites and bigger parasites. We cannot test the idea about the origin of sex directly, after the passage of so much time. However, there is now abundant research showing that, for organisms (such as some snails and fish) that are able to reproduce both sexually and asexually, the sexual forms predominate in habitats where parasites are common.
Neither sexual reproduction nor immune systems offer perfect defenses. We still get various parasite-induced afflictions. There are hookworms that latch on to intestinal walls and feed there; light infections create undernourishment and lethargy, heavy infestations can be lethal. The one-celled malaria parasite is a perennial threat, sometimes deadly, in tropical and semitropical areas. Another one-celled parasite can attack the immune system itself and is commonly lethal; a close relative of that little gem can eat up your face. The list of parasite-caused troubles is very long indeed.
So it is easy to think of parasites as evil, nasty little creatures, worthy only of extermination. In an increasing number of cases, however, there is another side to the story. Researchers have observed that human populations living in relatively parasite-free environments are more likely to develop allergies, asthma, and other immune disorders, such as ulcerative colitis and Crohn’s disease. In contrast, for those living where flukes and intestinal worms are a common affliction, these immune disorders are extremely rare. Similarly, in human populations where many people are exposed, early in life, to the parasitic organisms causing tuberculosis, malaria, hepatitis, and several other diseases, allergies are rarer than in populations with less early exposure to these parasites.
Normally, the immune system tries to deal with intestinal worms and flukes, as well as many disease-causing organisms. Where public health measure and modern medicine have reduced or eliminated these parasites, the immune system appears to over-react to otherwise harmless entities, such as pollen, mold, and cat dander, creating well-known allergies. In some cases, a good dose of worms can subdue these immune disorders. Eventually, instead of the worms themselves, it may be possible to use their proteins to keep the immune system under control. In the meantime, have some worms!
Parasites can be repulsive, debilitating, and deadly—but they have been part of our history for so long that perhaps we can’t do without them entirely!