An anthrax weapon aimed at a major city could kill at least 123,000 people even if every victim received treatment, experts have calculated.
Rapid care would be essential
US researchers have used a computer model to predict the devastation that would result from the launch of an anthrax bomb or missile on a city the size of New York.
The figures are based on what would happen if a bomb containing one kilogram of anthrax spores was dropped on a city of 10 million inhabitants.
The projected number of fatalities is based on the assumption that antibiotics would not be administered for 48 hours until the first symptoms appeared.
If it proved possible to distribute drugs more quickly, then the death toll could be substantially reduced.
However, they warn that inadequacies in the current US emergency response plan may make such a rapid response unlikely.
Lead researcher Dr Lawrence Wein, from the Graduate School of Business at Stanford University, California, said: "The first people develop symptoms within two days of exposure, and many more would develop symptoms over the next week.
"Our response needs to be measured in hours, not in days or weeks."
Five of the 11 people who inhaled anthrax during the 2001 attacks on the US postal system died despite intensive treatment by large teams of doctors.
The researchers recommend distributing anti-anthrax antibiotics such as Cipro in advance of any major attack.
If this was not possible, then the aim should be to distribute antibiotics to everyone infected within 12 hours.
In the case of an attack on New York City, that would mean supplying the drugs to 1.5 million people.
The only way to do this would be to increase the number of available health professionals dramatically.
The researchers estimate that to keep the death toll down to about 1,000, one health professional would be required for every 700 people in the affected population.
This could only be achieved by training non-emergency medical staff and making maximum use of military personnel and volunteers.
Dr Robert Spencer, an infection control expert at the UK Public Health Laboratory Service, told BBC News Online that the conclusions were similar to those reached by research carried out by the World Health Organization in 1970.
However, he said it was very difficult to determine what would happen should weapons grade anthrax be released on a city, not least because of weather patterns, and the complex effect of wind distribution in a built up area.
Dr Spencer said the only recorded case of anthrax release, from a Soviet installation in 1974, had resulted in surprisingly few cases of illness.
"It would be very difficult to disprove what they are saying," he said.
"My personal feeling is that anthrax is not a weapon of mass destruction, but a weapon of mass hysteria.
"Terrorists like bombs, they know what happens when they cause an explosion, and can make predictions based on that."
Dr Spencer also said that to stock up on vaccines and antibiotics to combat a possible anthrax attack would be to drain resources away from more certain demands for health care.
The research is published in the journal Proceedings of the National Academy of Sciences.