杏吧原创

While grim tales of coming plagues engage the popular imagination, biologist Marlene Zuk is focusing on the bright side of disease. Pathogens aren鈥檛 always our enemies, she says; in fact, they have helped shape our evolutionary history. As Zuk tells Peter Aldhous we ought to respect our complex 鈥� and sometimes comical 鈥� relationship with pathogens and parasites.

You call for greater appreciation of pathogens. What is there to appreciate?

This is somewhat tongue-in-cheek. We need to appreciate them not in the sense of praising, but in the sense of understanding the impact of parasites 鈥� and I include viruses, bacteria and fungi. When you take an art appreciation course, no one expects that you鈥檙e going to love all the paintings, but you鈥檙e going to see how art has affected life. Similarly, if you鈥檙e going to appreciate disease, you need to understand how it has affected life. It doesn鈥檛 mean you鈥檙e going to love diseases, or that you鈥檙e going to think it鈥檚 great to be sick.

Why have parasites and pathogens been such a powerful force in our evolution?

Because of the potential for co-evolution. When selection acts to produce an animal that is able to resist a drought, it leaves more offspring, and so you end up with a population that has, say, skin that is less likely to desiccate. But that doesn鈥檛 provoke the environment into becoming even drier. On the other hand, if defence against a disease evolves, there is selection pressure on the disease organism to evolve back. The tuberculosis bacterium might become better at penetrating your lungs, if your lungs have evolved a mechanism for resisting their entry.

You even suggest that aspects of human personality may have been shaped by parasites. What is the evidence?

I had fun with that, and it is speculative. Some wonderful work done several years ago looked at mice infected with a single-celled parasite called Toxoplasma. Their final host is a cat, so from the pathogen鈥檚 perspective, altering the behaviour of the rodent to make it more likely to be eaten would be an advantage. And it turns out that rodents infested with Toxoplasma are more likely to linger in areas that smell of cat. So there鈥檚 a change in personality, if you will. Then there鈥檚 some speculative evidence that people infected with the parasite are more likely to be in traffic accidents. It鈥檚 tempting to speculate that Toxoplasma makes you more willing to take risks.

So what is your favourite parasite?

There are nematode worms that get into crickets and grasshoppers, take control of part of the brain and make them seek out water sources they never ordinarily would. The parasites need water to be transmitted, so you find these dead crickets or grasshoppers completely split open after this enormous coiled worm has burst forth. If you want to be anthropomorphic, you can imagine that this cricket is just walking along thinking: 鈥淗uh! Taking a nice walk, lovely day, isn鈥檛 this fun? You know, I never really wanted to go down to that stream before, but suddenly it seems like a good idea.鈥�

You seem to have a particular appreciation for sexually transmitted diseases.

As humans we attach all this moral and ethical baggage to them, but they鈥檙e just another type of disease, and lots of animals and plants get them. I like the fact that scientists distinguish STDs from 鈥渙rdinary鈥� infectious diseases, which makes it seem that STDs are not nasty but somehow special. One of my favourites is called anther smut, which is a sexually transmitted fungus in plants. Ladybugs have sexually transmitted mites, and nobody talks about slutty ladybugs.

Sex features strongly in your book. You argue that parasites drove both the evolution of sexual reproduction and many aspects of animal sexual behaviour.

If you reproduce sexually, rather than through some form of cloning, your offspring are different from you. That can allow them to escape the parasite pressure that would potentially kill off a cloned organism. So parasites can help explain the evolution of sex.

鈥淧arasites can help explain the evolution of sex鈥�

With regard to mate choice, parasites help us solve a problem: why genetic variance doesn鈥檛 eventually disappear. Let鈥檚 say females prefer males with a particularly long tail. That鈥檚 all well and good, but after a while you run out of that variation and everybody has a long tail. But if a long tail correlates with the ability to fend off parasites, there鈥檚 a reason for tails to continue to grow. Because parasites are always evading their hosts鈥� defences, the hosts continually need to evolve defences against those evasion mechanisms.

You don鈥檛 like the 鈥渨ar鈥� metaphors commonly used to describe medicine鈥檚 approach to disease. What is wrong with them?

The problem is this idea that it鈥檚 possible to 鈥渨in鈥�. Antibiotic resistance is a great example. Many people hailed the invention of antibiotics as an end to bacterially caused disease. Yet evolution tells us that, since bacteria are living things, that鈥檚 very unlikely to be the end of it. I think antibiotic resistance is the best modern example of evolution anyone can think of. You don鈥檛 have to tell people about dinosaurs 鈥� this is happening in our hospitals every day.

You suggest that some of our concerns about emerging diseases are misplaced. You don鈥檛 seem so worried about avian flu, for example. Why not?

It鈥檚 not that I鈥檓 not worried about avian flu: it鈥檚 that we need to look at this in perspective. People get very concerned about a hypothetical risk, and are much less concerned about risks that actually exist, such as risks from antibiotic-resistant bacteria. I think it鈥檚 an absolutely essential exercise to think about how we鈥檙e going to respond if there鈥檚 a major flu epidemic. But is the average person going to get anywhere thinking about stocking up on Tamiflu? No. Should the average person be thinking about the correct use of antibiotics? Absolutely.

What do you make of the idea that a healthy immune system needs to be exposed to a sufficient number of pathogens?

Evidence suggests that if the immune system doesn鈥檛 have the appropriate stimuli at a very early age, things can go wrong later. For instance, you see a higher rate of allergies and asthma in children who grow up in households without pets or siblings. But not all children who grow up in ultra-clean environments develop these diseases.

So what causes this variation?

Vertebrates have an adaptive immune system that is very sophisticated. Maybe part of why this arose has to do with the need to manage the bacteria and other microbes living in our bodies, many of which are beneficial. People talk about the army of immune cells, but maybe we should think of them as office managers. They have to figure out who goes where and when, and who鈥檚 employed. They鈥檙e not fighting a war, they鈥檙e doing personnel maintenance.

Many people who write about the natural world take a reverential tone, but your book has many comic touches. How do colleagues react?

Anybody who teaches students recognises that being humorous is a good way to get people鈥檚 attention. My husband is waiting for me to get a review that calls the book a 鈥渓augh riot鈥� 鈥� but so far no luck.