Friday, June 17, 2011

Infectious diseases in our time

As regular readers know, we usually very much enjoy the BBC Radio 4 program, In Our Time.  We were quite looking forward to last week's show on the origins of infectious disease, as it promised to touch on a number of our interests, but we were sorely disappointed.

It should have been good, and indeed some of it was quite informative in a very basic way -- appropriately, for a show for a general audience.  The guests were geneticist Steve Jones, professor of infectious disease epidemiology Roy Anderson and professor of microbial genomics Mark Pallen.  Presenter Melvin Bragg doesn't know this field, but he had some good questions about such things as when infectious diseases first began to affect humans, where they came from and indeed still come from.

They begin with the history of the bubonic plague, and Steve Jones spoke about its origins and natural history.  He said that the growth of population led to the spread of disease, and often the rise of an epidemic coincides with change in proximity to an animal, such as the rat in the case of the plague.

As he has done before, Jones talked about three epochs of human history: the age of disaster, the 99% of human history in which most people were killed by tigers or cold; the age of disease, the rise of population from the beginning of agriculture 10,000 years ago, which brought the rise of epidemics; and the age of decay, at least in the western world, where we now die of old age rather than infection.

While he didn't talk about this in the current program, he has said previously that he believes that we were molded by selection during the first two epochs, but that natural selection will no longer be a force that molds us now that we mostly survive to reproduce and pass on our genes.

But what an odd thing to say, as infectious diseases are still a primary killer, especially of children in much of the world, and aren't going away anytime soon.  And, more and more diseases seem to have an infectious component, so it may indeed turn out to be much more influential now, even as we 'decay', than Jones suspects.

Further, even before large population sizes and agriculture, much of our species was probably exposed to continual varieties of parasites, infections in injuries, and the like.  We are immersed in all sorts of viruses and bacteria, even if the clearly infectious diseases, that we easily notice, have to do with large, dense populations and close association with large herds of domestic animals.

And of course new infections arise all the time, and indeed Roy Anderson believes that the coming century is going to see a huge increase in emerging diseases given global population size and the ease with which people globe trot and thus spread disease.  The idea that humans -- or any species until it goes extinct -- have stopped evolving is just wrong.  Yes, we adapt to changing environments to a great extent, but new alleles arise with every birth, and there will always be change in our species' genotypes.  What is new -- for the moment -- is the rate and nature of change.

The rate of change will be much lower than in most of our past, so long as population size is so large.  Likewise, the great amount of intercontinental mixing will slow down the rate at which specific genotypes can be screened by selection, because there will be so many more genotypes in the global mix.  Even mass deaths, by modern standards, will usually be trivial in relation to whole population sizes -- that is, won't make much of a population bottleneck that would purge variation.  Variation very specific to serious disease susceptibility could, of course, disappear fast, but positively adaptive genetic variants will be much slower to advance in frequency than in most of our past.

Medical care and other perks and protections of modern society (until such society collapses of its own excesses) will change the nature of selection, but we'll still evolve.  But where our evolution will be most interesting may be how the pathogens evolve and adapt to us, because they can evolve very fast, especially confronted with the strong selective effect of antibiotics and the like.

Bragg asked if it's possible to generalize about where most viruses come from.  Anderson said that homo sapiens are 1/2 million years old (though it's not clear where that figure came from), and that we acquired all our infectious agents from wild animals before we domesticated animals, and then acquired disease from livestock.  Usually infection traveled in one direction, animals to humans, though several examples of infection going the other way did come up during the program -- we gave TB to cattle, and leprosy to armadillos, for example. An interesting twist, since sometimes now these pathogens mutate and get transferred back to us!

Jones was asked about the effect of animals on epidemics.  He said, as Anderson did, that indeed most diseases come from animals, but he then said that in fact the most dangerous animals are our closest relatives -- which is another odd thing to say on the face of it, as he'd just told us that the bubonic plague, one of the biggest killers ever, came from fleas via rats, and we get malaria from mosquitoes. We get relatively few such diseases from primates, so whether pigs and chickens are our 'closest' relatives in this context isn't clear.  But it is clear that population structure and habitat are important.

Jones also said that only recently, at the beginning of the 20th century, has medicine stopped killing more people than it cured.  But this is meaningless, as the denominator, the number of people who access medicine and what they go for, has changed dramatically.  It's comparing apples to, to ugli fruit.  Without affordable health care, few would have gone to the doctor for a common cold in the 19th century, but only when they were already deathly ill.  And probably most afflictions people see their doctors for now are self-limiting anyway, or at least aren't fatal, so of course a much smaller fraction of the people who see doctors now die.  Which is not to say that medicine hasn't improved, because of course it has.  

It is not at all clear that in the first age of humanity disaster killed more people or shortened life more than parasites of various kinds, nor that, overall, we're not still in the age of infection.  Certainly we suffer the slings and arrows of aging these days, but this is less clear outside of the 'developed' world, and indeed some of these slings and arrows may be laced with pathogens.  Also, in evolutionary thinking one might argue that even with rampant infection still so widespread, there are vastly more people alive today than ever before.  So whatever the changes, from an evolutionary point of view they've been good for our species, no matter how many suffer from whatever causes.

Pallen ends the program by saying that after they finish recording he wanted to talk with Jones about which theory was more important in terms of our understanding of the world and how we affect it, evolutionary theory or the germ theory.  What he meant, we assume, was that understanding how germs infect and kill, and their behavior in populations has allowed us to control them in ways that evolutionary theory could never do.  But in fact much of our immune system evolves, along with parasites, in the bodies of infected people.  So it seems an odd kind of comparison.
Evolutionary theory works far better and more clearly for such rapid adaptive changes, in our parasites, than those that happen in ourselves.  But asking whether evolutionary theory or the germ theory is more important is like asking which is more important, brakes or drapery.  They are both important  in their own way.  And evolutionary theory is a framework for understanding life, not a prescription.

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