Filed at 12:01 p.m. ET

Amid heightened concern about bioterrorism, a new study offers a rough answer to one question: How many people would a smallpox victim likely infect during an outbreak of the deadly disease?

The answer: four to six on average, say researchers at England's Centre for Applied Microbiology and Research in Salisbury. They report their conclusion in the Dec. 13 issue of the journal Nature.

The director of an American research center challenged the number, which was derived from a mathematical analysis of data dating to the 18th century. Recent prior estimates have ranged from about 1.5 to more than 20.

Tara O'Toole, director of the Johns Hopkins University Center for Civilian Biodefense Studies, questioned whether the latest number was in any way predictive or useful. O'Toole, who did not participate in the study, said an immense range of variables likely would to come into play in any modern-day smallpox outbreak.

``Is the (infected) person a school child, is the person an elderly live-at-home, is the person someone who travels all over the world?'' she said. ``You can model it with different assumptions and see where it goes and in what directions, but the notion we can actually identify the transmission rate (of smallpox) is lunacy.''

In developing the mathematical model, researchers Raymond Gani and Steve Leach culled a variety of historical data from five 18th century smallpox outbreaks; a 19th century outbreak in London; a 1972 outbreak in Kosovo; and various European outbreaks between 1958 and 1973.

The study took into account the proportion of the population susceptible to the disease, socio-economic factors, and the varying levels of immunity to the disease as a result of vaccinations.

Gani and Leach said their findings show smallpox would be transmitted at a ``relatively modest'' rate compared to measles or chickenpox. But, they noted, ``significant smallpox epidemics could occur, particularly if there were delays in detecting the first cases or in setting up effective public health interventions.''