Smallpox is highly infectious and is transmitted, like colds and flu, by sneezing, coughing and by people shaking hands after wiping their mouths.
The first priority, in the event of an attack, would be to isolate the victims to prevent them passing on the infection. That, of course, would depend on rapid identification.
The disease causes a rash and later pustules develop on the skin, which are easily recognisable. But in its early stages the symptoms of smallpox are fever and aches, similar to flu.
Only two stockpiles of the virus remain, in laboratories in the US and Russia, and there are fears that some of it could have fallen into terrorist hands.
Like all viruses, smallpox would not be susceptible to antibiotics; they are only effective against bacteria, including anthrax.
Smallpox is said to carry a 40 per cent mortality rate but the level varies according to natural immunity.
Dr Rick Hall, of the Centre for Applied Microbiological Research, Porton Down, said: "The easiest route of entry into the body is via the respiratory tract. Inhaling the virus as an aerosol would be the best way [of delivering it as a weapon]."
Dr Hall said that growing a virus in the laboratory would demand more skill than cultivating bacteria because a virus can only replicate inside living cells. There would have to be a means of getting the virus into the living cells, and out again, and it would have to be stored so that it wouldn't degrade – for example, by freezing it.
By contrast, anthrax is easy to cultivate in a fermentation brew, like making beer, and is easy to store in its spore form.
That is what makes anthrax more attractive as a biological weapon to terrorists.
Dr Hall said the risk attached to any biological threat depended on its ease of manufacture and distribution and its transmissibility, lethality and the availability of medical treatments to combat it.
"It is a combination of all these factors that add up to the whole," he said. Ebola or Marburg viruses – different strains of haemorrhagic viruses which cause internal bleeding – could also theoretically be employed by terrorists in an attack.
They are lethal and contagious, but as there is no vaccine against them they would be hazardous to collect and to work with.
Periodic outbreaks in sub-Saharan Africa have killed scores of people in poor, rural areas but it is difficult to predict what the effects or an outbreak would be in a healthy, Western population.Reuse content