WHY SHOULD scientists all over the world be so concerned about the appearance of one fatal case of smallpox in England? After all, smallpox has virtually disappeared from the West Hemisphere, where only five people are known to be contracted it in the last 30 years. Its disappearance prompted the United States and other Western nations to end their smallpox vaccination programs in the early 1970s and stockpile their vaccination supplies. And the World Health Organization's anti-smallpox campaign in developing countries has eradicated the disease from all continents except Africa, where the last known case occurred in 1977. If no further case occur by 1980, international health officials will declare smallpox to have eliminated from the world. When that happens, only research laboratories will have the smallpox virus - and that is precisely the cause of the scientist's concern.

They fear that smallpox, highly infectious and lethal, could erupt some time in the future through accident or carelessness in a laboratory and spread rapidly through a population no longer immunized against it. In fact, the last two smallpox cases in Western countries (both in England) were caused by leaks from a laboratory. The latest incident occurred last month, when an employee of an English medical school who worked above a laboratory containing smallpox virus contracted the disease. The woman died this week.

It was to minimize the possibility of just such an incident that the World Health Organization two years ago asked the 76 laboratories throughout the world that then had smallpox virus on hand to either destroy it or transfer it to one of five designated disease-control centers (in Russia, Japan, the Netherlands, England, and the Center for Disease Control in Atlanta, Ga.). Thus far, all but 14 (including the English medical-school laboratory) have done so.

The two American laboratories that have not disposed of the virus are both in the Washington area. One is a U.S. Army research facilty. The other is a private research laboratories that contains one of the world's major archival collections of microorganisms. Neither facility does any research involving the smallpox virus. Officials of both laboratories (they are considering the WHO reqeust) have said that, despite the disease'e elimination as a health problem, the smallpox virus should be kept by some laboratories for reference purposes. Doing so, they contend, will make it easier in the future to determine quickly if a suspicious rash actually is a case of smallpox. No one disputes that view. In fact, that's one reason for WHO's sound proposal to gather all existing smallpox virus stocks at the five designated centers. The real question is whether the laboratories keeping the virus should be the five designated centers, where it will be stored under uniformly exacting standards, or a collection of public and private laboratories subject to no single clear standard. We much prefer the first alternative. There is a need to reduce to the absolute minimum the risk of another laboratory leak abroad or in this country. How ironic it would be if this horrible disease, wiped out by the advances of modern science, were to erupt again because of a scientist's mistake.