The new virus has killed around 260 people since last November. "In that period of time, tens of thousands could be expected to have died from flu and pneumonia," said Dr Peter Marsh, a social psychologist and director of the social issues research centre at Oxford.

"We are used to health scares, but this has taken on a whole new scale," he said. The calculus of risk and fear is a puzzle for public health authorities. Malaria, which kills a child every 30 seconds in Africa, is a real threat to half the world. Tuberculosis is on the increase almost every where; poliomyelitis cases have suddenly made a steep rise in India, even though the virus is almost extinct.

In the last 20 years more than a score of newly identified infections - from deadly Ebola fever to Lyme disease caught by ticks carried by deer - have caused flurries of public alarm.

Humans tend to worry more about the unfamiliar and the improbable. "It's foreign, it's eastern," said Dr Marsh. "The fact is that 260 people have died. But for every Chinese person who has died, 10 million have not. In an ordinary rational world, that sounds like quite good odds, but not in this context. In this country, every year, 1,500 people are killed falling down the stairs. The implication would be that people should only be allowed to build bungalows."

The virus has been described as a "time bomb". There has been talk of it "mutating".

"Once you have that kind of imagery," said Dr Marsh, "then rational consideration, rational decision-making really goes out of the window."

Mary Burgess, a consultant clinical psychologist at University College London hospital, saw a parallel with the early days of HIV. "This [Sars] is a disease that is caught very specifically; you have to have specific contact with people. But with phobias, people start to avoid going on certain transport, or start avoiding certain groups of people.

"They are trying to contain their anxiety, and it can become phobic."