Scientists long anticipated that an influenza virus will spread - as now appears to be happening in Asia - from wild birds to humans, causing a pandemic. But few predicted last year's SARS epidemic, a pneumonia caused by a coronavirus . Prior to 2003, coronaviruses ranked very low on the scale of important human diseases, mostly being associated with the common cold. But we have long known that coronaviruses can cause dreadful diseases in domestic animals. We simply haven't learned our lesson.
Indeed, events of the last couple of decades - the AIDS viruses transferring from monkeys to man, followed by their global spread, being just one example - should have convinced us that, where diseases are concerned, the unexpected will happen. After all, the "jumping" of viruses from wild animals to humans is more common than we like to think.
All types of virus strains or variants exist - somewhat akin to dogs existing as different breeds. Veterinarians and farmers have long known of a coronavirus that can kill 90% or more of young pigs. Less well known is the fact that cats and dogs are infected by a coronavirus that can also cause disease in pigs.
The cat coronavirus can cause lethal abdominal disease in cats, while some strains of the chicken coronavirus cause kidney disease rather than just bronchitis. There is simply no room for complacency where viruses - or bacteria, for that matter - are concerned.
Evolution - whether of microbes or humans - involves pushing at the margins, going a little bit further than yesterday or last year, driven by the need to find additional sources of food. Humans, unlike other creatures, may also desire more of everything. Whatever the reason, we invade other creatures' space by, say, cutting down forests.
As we come close to other animals, their viruses come closer to us. The outcome can be the same if we trap, enclose, and trade wild animals, such as civet cats, which seems to be the scenario that led to SARS in humans.
When a virus has been associated with its host for a very long time, it mutates to a form that grows well without killing its host, which would be a pointless outcome from an evolutionary standpoint. A well-understood example is that of influenza viruses in wild fowl, where the virus causes infections that are hard to detect.
But if an influenza virus jumps from, say, ducks to chickens, there may be lethal consequences. The virus is not well adjusted to the chicken, and some strains of the virus "go berserk."
It is possible that the SARS coronavirus does not cause serious disease in its natural, wild animal hosts. Our problem arose when we enabled the virus to move from its normal home into us, by "farming" civet cats and other animals.
Many human viral diseases have not been with us very long, perhaps only ten to twenty thousand years. Most probably, the viruses came from the wild animals in whose environment we were encroaching. In other words, the SARS outbreak was not a novel event.
Chinese researchers have detected antibodies to SARS-like viruses in 2% of people from whom blood samples were taken in 2001 - a year before the disease first occurred. Many animal traders tested in 2003 in a market in the epicenter of the SARS outbreak also had the antibodies - again with no history of disease. This shows that the SARS virus has jumped species, from animals to man, on other occasions, with benign consequences.
The difference in late 2002 was, perhaps, that people were infected with a variant that "went berserk" and grew too extensively in humans. It was bad luck. It is also possible that - again by chance - a benign SARS virus from a civet cat mutated after it infected people, becoming highly virulent.
Both aspects of this scenario - a virus jumping to a strange host and then mutating to a devastating form - are familiar. Waterfowl commonly spread influenza viruses to chickens, although the outcome usually is not serious. On occasion, however, the virus mutates to a lethal form that can kill almost 100% of its victims. It is a highly virulent influenza virus such as this that is devastating chickens in parts of Asia.
That influenza virus has also "jumped" from the chickens to those who look after them, killing some people. The influenza viruses established in humans for several decades originated from bird viruses. The chronic fear of health authorities is that, by chance, another avian influenza virus will mutate and spread from person to person to spawn a global pandemic.
Human SARS might never have spread to the extent that it did (killing 800 people and devastating economies) if the problem had been openly acknowledged, with the World Health Organization involved from the outset. Similar failings have exacerbated the consequences of today's bird-origin influenza epidemic in Vietnam, Thailand, and elsewhere. Some countries that have the greatest reservoirs of viruses with "jump potential" are ill equipped politically, socially, and institutionally for the world to have confidence in them.
Humans will continue to push against our existing boundaries, whether out of necessity, curiosity, pleasure, or greed, so more virus jumps will undoubtedly occur. We must trust the WHO and its associates to come to our rescue, as it did so admirably with SARS. But those involved in human and veterinary medicine must no longer remain aloof from one another. Recent events have made crystal clear what we already knew - that human and animal viruses are not mutually exclusive.