Coronaviruses are versatile in other ways too, with some strains able to infect more than one species. A study two years ago showed that a coronavirus isolated from cattle could also infect baby turkeys, though not, curiously, baby chickens. "Coronaviruses may be much more promiscuous than we originally thought," says Linda Saif, a veterinary scientist and virologist at Ohio State University.
Scientists have only begun to learn the rules of engagement the SARS coronavirus follows. Like many of its kin, it appears to be a lung-and-gut bug; people die from lung damage; about one-fifth of its victims also have vomiting and diarrhea. But SARS-CoV behaves unlike many respiratory viruses. For one thing, the disease it causes develops slowly. Also, there’s an almost miraculous sparing of children. In the recent SARS outbreak, few children became ill and none under age 16 died. Scientists don’t yet know why.
If SARS-CoV entered the human population from animals, it is by no means the first virus to make the jump between species. Measles, which has afflicted human beings for at least 2,000 years and still kills more than 700,000 people annually (mostly children), is caused by a virus whose closest relative causes rinderpest, a disease of cattle. The domestication of animals brought human beings and bovids together in large numbers, and some of the herd’s pathogens adapted to life in the herders. A similar leap ages ago may have introduced human populations to the smallpox virus, which has since been eradicated.
Perhaps the most important question about SARS—is it with us forever?—can’t yet be answered. According to preliminary reports, some exotic mammals in southern China that are caught and sold for food (including the masked palm civet) harbor a coronavirus identical to SARS-CoV with an important exception: the animal virus’s RNA has an additional 29 nucleotides, or chemical subunits. The similarity suggests that the SARS virus arose from the animal virus. If those 29 missing nucleotides hold the key to the emergence of SARS-CoV, its future may depend on how frequently that particular genetic deletion occurs. It may not happen again for decades, or centuries. Or it could happen next year. But even if the virus’s genetic material changes frequently, future epidemics may possibly be prevented merely by keeping people away from palm civets and other infected species.
Alternatively, SARS may behave like Ebola hemorrhagic fever, which appears periodically. Ebola emerged in 1976 in simultaneous outbreaks in Zaire and Sudan. The virus strikes in Africa every few years, killing 50 to 90 percent of the people it infects, and then vanishes. Despite great effort, scientists still haven’t found the natural animal host or reservoir for Ebola virus, and that makes it harder to prevent periodic outbreaks.
By early July, the W.H.O. declared that the outbreak was over. At last count, 8,399 people in 30 nations had been identified as "probable" SARS cases and 813 of them had died.
Of course, even though the SARS epidemic is officially over, the virus may actually still be with us. A few survivors are known to have carried it for months and may be contagious. It’s also conceivable that a handful of people with the disease have escaped detection. For those reasons, some medical experts believe that only a vaccine can rid humanity of SARS for certain. Making and testing one will require at least three years of work, says Gary Nabel, director of the vaccine research center at the National Institute of Allergy and Infectious Diseases. (The same is likely to be true for anti-SARS drugs.) Even so, animal coronavirus vaccines have a spotty record. Some provide only transient protection. Others, like the vaccine against feline coronavirus, can even worsen an infection under some circumstances. Until good drugs and an effective vaccine are available, the best approach to preventing the global spread of the disease is decidedly old-fashioned: identifying infected persons, isolating them until they recover and quarantining people who’ve had close contact with the victims. Those measures, applied assiduously in recent months and in many nations, appear to have accomplished something nearly unheard of in the history of medicine—halting an epidemic respiratory infection, at least temporarily.
For his part, the CDC’s Zaki is betting on SARS’s return. "I don’t see any reason why it shouldn’t come back," he says. "We can learn from history. If it happened once, it can happen again."
The flip side of such fatalism—or is it realism?—is that despite some predictions that the emergence of SARS augurs a new millennium of ever-accumulating human scourges, nothing about it is foreordained. We shouldn’t forget that thanks to sanitation, affluence and medicine, in many parts of the world far more infectious diseases have retreated than have emerged in the past century. The appearance of SARS, like so many important historical events, was the product of dozens, or hundreds, of small occurrences, many of them chance. It was neither inevitable nor entirely unexpected. It’s just what happened.