Strange guests with deadly habits

He must have regretted his words. On December 4 1967, the United States surgeon general announced that infectious diseases had been conquered. The old scourges of bubonic plague, smallpox and malaria were overcome, and new vaccines and antibiotics promised to sweep away typhoid, diphtheria, tuberculosis and polio. Medicine could now concentrate on cancer and the other diseases that are typical of an affluent and long-lived society.

The greatest successes in modern medicine are indeed in fighting the infectious diseases, if success is measured in years of life saved. Perhaps the pinnacle of this achievement was reached ten years after the surgeon general's ill-fated speech, with the eradication of smallpox by vaccination. But the greatest success in medicine has engendered the greatest arrogance - the greatest hubris, as Frank Ryan calls it in this book. The naive belief that infectious diseases were in principle beaten has been banished by the spread of antibiotic-resistant bacteria and by the now fashionable study of "emerging infectious diseases". A succession of apparently new infectious diseases have been recognised in the past few years: of these, Aids has done more than any other to overturn the complacency of both doctors and public on the subject. It is the purpose of this book to alert more people to the possibility that a new lethal pandemic virus could arise, and kill all or most of the human population. If it is right, this book should be of interest to anyone concerned with infectious diseases, or indeed with the survival of the human race. Is it reasonable? Could it really happen?

The term "emerging infectious disease" has been used, I believe, mainly as a rhetorical device to persuade people that something really new is happening, to make them take the problem seriously. Yet the truth is that new infectious diseases have probably emerged into human society throughout its existence: some become endemic in humans, while others flare up occasionally and then vanish.

Only two things, I suggest, are really new. The first is our medical hubris, our conclusion that we can outwit infectious disease, to which HIV and the other emerging infectious diseases have dealt a rude slap in a face. The second is the extraordinary advance of science in this century, which has made it possible to detect and identify viruses and bacteria which previously went unrecognised.

Ryan tells a remarkable tale of such a discovery in the opening chapters - the best part - of this book. A previously fit young man who lived on a Navajo reservation in New Mexico died in 1993 from a rapidly progressive pneumonia. The infection spread to his family and other contacts, and bewildered doctors by its high mortality rate and its unfamiliarity. For a while, pneumonic plague was suspected as the cause. The tension is kept up through six chapters in this pacy description, because the identity of the infection is withheld to the end. Then in an astonishing denouement, Ryan describes how, within 26 days of the first death, a laboratory at the Centers for Disease Control in Atlanta identified some of the genes of the infectious agent. This showed that it was a "new" virus, related to one that caused devastating outbreaks of haemorrhagic fever in the Korean war in the l950s.

Such rapid identification of this new virus, a Hantavirus, was an extraordinary achievement, and yet it was of more immediate importance to find where the virus emerged from. It turned out to be harmlessly endemic in one of the commonest rodents in North America, the deer mouse, which had just undergone a population explosion in the district. It is now clear that most apparently new viral diseases are acquired by humans from animals, and the other case studies in Ryan's book illustrate this point. HIV was almost certainly caught by people from monkeys, not through unnatural sexual practices but through butchering infected monkeys. Rodents present a particular danger because they are abundant and often live in or near human dwellings; Lassa fever virus is another notorious emerging virus infection that is harboured, again harmlessly, by rats and only occasionally transferred to humans.

Harmlessly? How can these viruses be harmless in rodents and yet kill many infected humans? This difficult question preoccupies Ryan in the second half of the book, and leads him seriously astray into some mystical speculation on evolution. Ryan's argument runs like this. A virus that has long been present in an animal population becomes less virulent to its host, because natural selection favours the survival of animals with a degree of resistance, and because the virus will spread and survive better if it too evolves to kill fewer hosts. This much seems reasonable; there is a grain of truth in it, although it has been well established for many years on both theoretical and experimental grounds that, as Ryan admits, an infectious agent does not always evolve to benignness.

But why should an infection that has become benign in one species remain virulent in others? Here Ryan really gets into deep water. He says that viruses have "genomic intelligence", that is, a capacity of the genetic make-up to be "... both receptive and responsive to nature". At first sight this seems to be no more than a metaphor for the logic of natural selection. But Ryan deduces a subtler, more sinister scheme underlying the co-existence of a virus and its host species which, it seems, "is not trying desperately hard to shake off the virus". He claims that the hosts tolerate the endemic infection because the virus retains its virulence and lethality to other species which might invade the territory of the virus's primary host. The "aggressive symbiosis" promotes the survival of both virus and host.

This is teleology at its most brazen, and it will not do. Evolution is blind, and cannot anticipate events: the virus cannot plan for the future. Besides, if the virus was really intelligent, surely it would establish a harmless infection in the new host as well, to maximise its chances?

I do not wish to belittle the ability of viruses to survive and to adapt: their success is all too plain. (Ryan calls this belittlement "manderiding ...to compare the attribute of another life form to the human equivalent in order to deride it"). That is not the point. The main criticisms are these. First, it is not necessary to invoke anything other than natural selection to explain the evolution of viruses. Yet it is not possible - Ryan does not try - to envisage a mechanism by which a virus could specifically maintain its virulence to a species that might invade its territory. Second, the concepts of "aggressive symbiosis" and "genomic intelligence" have no scientific meaning, and serve only to obscure the path of future enquiry with anthropomorphism and metaphor.

It is a pity that such muddled thought on evolution is mixed with an interesting account of the important subject of emerging viruses. The conclusion is correct, that we must be aware of and prepared for the danger of a new lethal epidemic infection, probably caught from wild animals. But the evolutionary reasons presented here are specious.

Charles R. M. Bangham is professor of immunology, Imperial College School of Medicine, St Mary's, London.