Richard Dawkins - The Magic of Reality

Parasites often attack in large numbers, as when our body suffers a massive infection with a flu or cold virus. Parasites that are too small to see with the naked eye are often called ‘germs’, but that’s rather an imprecise word. They include viruses, which are very very small indeed; bacteria, which are larger than viruses but still very small (there are viruses that act as parasites on bacteria); and other single-celled organisms like the malarial parasite, which are much larger than bacteria but still too small to be seen without a microscope. Ordinary language has no general name for these larger singled-celled parasites. Some of them can be called ‘protozoa’, but that’s now rather an outdated term. Other important parasites include fungi, for example ringworm and athlete’s foot (big things like mushrooms and toadstools give a false impression of what most fungi are like).

Examples of bacterial diseases are tuberculosis, some kinds of pneumonia, whooping cough, cholera, diphtheria, leprosy, scarlet fever, boils and typhus. Viral diseases include measles, chickenpox, mumps, smallpox, herpes, rabies, polio, rubella, various strains of influenza and the cluster of diseases that we call the ‘common cold’. Malaria, amoebic dysentery and sleeping sickness are among those diseases caused by ‘protozoa’. Other important parasites, larger still – large enough to be seen with the naked eye – are the various kinds of worms, including flatworms, roundworms and flukes. When I was a boy living on a farm, I would quite often find a dead animal like a weasel or a mole. I was learning biology at school, and I was interested enough to dissect these little corpses when I found them. The main thing that impressed me was how full of wriggling, live worms they were (roundworms, technically called nematodes). The same was never true of the domesticated rats and rabbits we were given to dissect at school.

The body has a very ingenious and usually effective system of natural defence against parasites, called the immune system. The immune system is so complicated that it would take a whole book to explain it. Briefly, when it senses a dangerous parasite the body is mobilized to produce special cells, which are carried by the blood into battle like a kind of army, tailor-made to attack the particular parasites concerned. Usually the immune system wins, and the person recovers. After that, the immune system ‘remembers’ the molecular equipment that it developed for that particular battle, and any subsequent infection by the same kind of parasite is beaten off so quickly that we don’t notice it. That is why, once you have had a disease like measles or mumps or chickenpox, you’re very unlikely to get it again. People used to think it was a good idea if children caught mumps, say, because the immune system’s ‘memory’ would protect them against getting it as an adult – and mumps is even more unpleasant for adults (especially men, because it attacks the testicles) than it is for children. Vaccination is the ingenious technique of doing something similar on purpose. Instead of giving you the disease itself, the doctor gives you a weaker version of it, or possibly an injection of dead germs, to stimulate the immune system without actually giving you the disease. The weaker version is much less nasty than the real thing: indeed, you often don’t notice any effect at all. But the immune system ‘remembers’ the dead germs, or the infection with the mild version of the disease, and so is forearmed to fight the real thing if it should ever come along.

The immune system has a difficult task ‘deciding’ what is ‘foreign’ and therefore to be fought (a ‘suspected’ parasite), and what it should accept as part of the body itself. This can be particularly tricky, for example, when a woman is pregnant. The baby inside her is ‘foreign’ (babies are not genetically identical to their mothers because half their genes come from the father). But it is important for the immune system not to fight against the baby. This was one of the difficult problems that had to be solved when pregnancy evolved in the ancestors of mammals. It was solved – plenty of babies do manage to survive in the womb long enough to be born. But there are also plenty of miscarriages, which perhaps suggests that evolution had a hard time solving it and that the solution isn’t quite complete. Even today, many babies survive only because doctors are on hand – for example, to change their blood completely as soon as they are born, in some extreme cases of immune-system overreaction.

Another way in which the immune system can get it wrong is to fight too hard against a supposed ‘attacker’. That is what allergies are: the immune system needlessly, wastefully and even damagingly fighting harmless things. For example, pollen in the air is normally harmless, but the immune system of some people overreacts to it – and that’s when you get the allergic reaction called ‘hay fever’: you sneeze and your eyes water, and it is very unpleasant. Some people are allergic to cats, or to dogs: their immune systems are overreacting to harmless molecules in or on the hair of these animals. Allergies can sometimes be very dangerous. A few people are so allergic to peanuts that eating a single one can kill them.

Sometimes an overreacting immune system goes so far that a person is allergic to himself! This causes so-called auto-immune diseases ( autos is Greek for ‘self’). Examples of auto-immune diseases are alopecia (your hair falls out in patches because the body attacks its own hair follicles) and psoriasis (an overactive immune system causes pink scaly patches on the skin).

It is not surprising that the immune system sometimes overreacts, because there’s a fine line to be trodden between failing to attack when you should and attacking when you shouldn’t. It’s the same problem we met over the antelope trying to decide whether to run away from the rustle in the long grass. Is it a leopard? Or is it a harmless puff of wind stirring the grass? Is this a dangerous bacterium, or is it a harmless pollen grain? I can’t help wondering whether people with a hyperactive immune system, who pay the penalty of allergies or even auto-immune diseases, might be less likely to suffer from certain kinds of viruses and other parasites.

Such ‘balance’ problems are all too common. It is possible to be too ‘risk averse’ – too jumpy, treating every rustle in the grass as danger, or unleashing a massive immune response to a harmless peanut or to the body’s own tissues. And it is possible to be too gung-ho, failing to respond to danger when it is very real, or failing to mount an immune response when there really is a dangerous parasite. Treading the line is difficult, and there are penalties for straying off it in either direction.

Cancers are a special case of a bad thing that happens: a strange one, but a very important one. A cancer is a group of our own cells that have broken away from doing what they are supposed to do in the body and have become parasitic. Cancer cells are usually grouped together in a ‘tumour’, which grows out of control, feeding on some part of the body. The worst cancers then spread to other parts of the body (that’s called metastasis) and eventually often kill it. Tumours that do this are called malignant.

The reason cancers are so dangerous is that their cells are directly derived from the body’s own cells. They are our own cells, slightly modified. This means the immune system has a hard time recognizing them as foreign. It also means it is very difficult to find a treatment that kills the cancer, because any treatment you can think of – like a poison, say – is likely to kill our own healthy cells as well. It is much easier to kill bacteria, because bacterial cells are different from ours. Poisons that kill bacterial cells but not our own cells are called antibiotics. Chemotherapy poisons cancer cells, but it also poisons our healthy cells because they are so similar. If you overdo the dose of the poison, you may kill the cancer, but not before killing the poor patient.