On the other hand, if the masked-palm-civet hypothesis is correct, it is somewhat encouraging that the coronavirus isolated from the civets by Hong Kong scientists had 29 additional bases compared with the strain isolated from patients, because it implies that SARS-CoV was the result of a one-time genetic deletion from the virus found in civets. This deletion may have been a freak event that will not recur, and if it doesn't, neither will SARS.

Another major area of uncertainty is the effects of climate and seasonality. Many respiratory infections display seasonality: that is, the number of cases increases during the winter, at least in temperate-zone areas, then decreases during the summer (hence the term "cold and flu season"). This is true of influenza, and many cold-causing viruses, including the human coronavirus CV-229E.

The reasons for this seasonal variation are not clear, although there are many theories. It might be because people congregate indoors during the winter. Or the still, dry air in heated houses might allow droplets of respiratory viruses to persist for a longer period, presenting a greater risk of infection. Or a relative lack of microbe-killing solar radiation in temperate zones during the winter might keep airborne viruses alive for a longer time. In southern China, there is a tradition of eating wild-animal dishes during the winter months, due to the belief that extra nutrition is needed during the winter; researchers have speculated that this practice may contribute to the origins of many epidemics in this region.

Whatever the reasons, the seasonality factor lies behind the concern of many researchers that SARS may have been beaten back as much by the summer weather as by the specific countermeasures taken against it. Scientists have noted that SARS seemed to prosper most in developed countries during cooler weather, and withered in less-developed countries during warmer weather. Since it seems unlikely that northern Vietnam and Guangdong province had superior infection-control measures to, say, Singapore and Toronto, the greater use of climate control - which inadvertently created a more hospitable climate for SARS-CoV - may be partially to blame for the epidemic's greater extent in the latter two cases. In a specific investigation of the climate issue, a group of Chinese researchers including scientists from East China Normal University identified a set of weather conditions that seemed permissive for SARS outbreaks. These conditions included a daily average temperature of 13-17 degrees C, with six to eight days of low temperature variation and poor air quality preceding the outbreak. Although these data are only correlational, health authorities may be able to use them to identify time periods, and regions, that are at heightened risk for a new outbreak of SARS, and implement precautionary measures.

If a new outbreak of SARS did occur, health-care workers would need to identify patients with SARS quickly and separate them from cases of "normal" pneumonia. The second, "flare-up" outbreak in Toronto, which occurred after the initial outbreak there was believed to be under control, was caused by a failure to recognize that an elderly patient with pneumonia actually had SARS. Prevention of future outbreaks hinges on the wide availability of a fast, reliable test for SARS. Good polymerase chain reaction (PCR) based tests now exist; the prototypes developed by research groups in the spring have now been joined by commercial kits from firms such as Abbott Labs and Roche, which even donated 2,000 of its test kits to hospitals in the Asia-Pacific region.

But it is one thing for a research hospital in Hong Kong or Singapore to have these tests; it is another for a rural health clinic in the Chinese or Vietnamese hinterland to have them. Whether the kits will actually be available when and where they are needed remains to be seen.

A final unknown factor is hygiene. The extreme measures seen throughout East Asia - massive deployments of disinfectant, hand wipes, masks, sterilizing gels, avoidance of crowded areas, and so on - cannot be continued permanently, nor should they be. But it is conceivable that the SARS epidemic may cause a long-lasting change in the behavior of the East Asian population in a way that makes the area less vulnerable to respiratory illnesses. All across the region, the public is more aware that such practices as frequent hand-washing, avoiding spitting and so on are crucial in avoiding disease. The benefits of a clear cultural change in this area could be felt for generations to come. On the other hand, if the public promotion of hygiene takes the form of a short-lived "campaign", the effects of which disappear without a trace the moment the campaign is over (as is the usual practice in the officially "socialist" countries of Asia), the benefits could be transitory. 

It is impossible, overall, to say whether SARS will return. But the preponderance of the evidence suggests that it will not, at least not this year. The most important reason for this conclusion is that the virus almost certainly originated in a wild animal, and the Chinese government's heightened scrutiny of the wild-animal trade makes another cross-species transmission unlikely (although a ban on civet sales imposed in May has reportedly been lifted). If SARS-CoV originated in a single mutational event that made the virus able to infect humans, as some scientists believe, a recurrence is even less likely.

(Note: As this article went to press, there was a confirmed SARS case in Singapore. But the case does not constitute a "recurrence of SARS" in the epidimiological sense discussed here. The infected man was a 27-year-old virologist at the National University of Singapore, who visited an Environmental Health Institute (EHI) lab, where SARS had been studied, three days before becoming sick. The presumptive hypothesis was that he had been infected during his visit to the EHI lab, and this was quickly confirmed. Fortunately, the man, who has already recovered, did not infect others during his illness. The Singapore authorities responded to the incident by shutting down both labs, quarantining the workers, and inviting outside experts from the WHO and US CDC to review safety precautions at the two laboratories.) 


If SARS does return, will it be more or less severe in terms of symptoms? In this case, the answer seems fairly clear-cut: it will be equally severe, at least for the next few years. Most infectious diseases display gradual decrease in severity with time, as the host population grows more resistant and the pathogen itself adapts to its host by becoming less virulent. But there is no evidence yet that this has happened with SARS.

SARS seems to be unusual among human diseases in that infection invariably causes the SARS symptoms. Researchers have not been able to find any "asymptomatic carriers", ie, people who are carrying SARS-CoV, but have not become ill. Although this is bad news for anyone who is exposed to SARS-CoV, it is actually good news for the public-health authorities, because it means carriers of SARS-CoV can quickly be identified and isolated. Outbreaks are easier to suppress when asymptomatic carriers do not exist, because, like the notorious "typhoid Mary", they constantly start new outbreaks while remaining well themselves.

In addition, scientists were surprised to find that SARS-CoV underwent very few genetic changes as the epidemic proceeded. This was unexpected because the virus keeps its genes in the form of ribonucleic acid (RNA), which has a greater tendency to mutate than deoxyribonucleic acid (DNA). One implication of this finding is that the SARS virus may take longer to lose its virulence than some other well-known pathogens, eg, the influenza virus. Thus, if SARS does return, we unfortunately have to expect that it will remain as lethal as before.