The World Health Organisation is not a body noted for its hyperbole. So when it issued a stark warning last week about a new disease which it described as "a worldwide health threat", a shiver of fear ran through public-health authorities around the globe. The new disease, which has been given a new name - severe acute respiratory syndrome (SARS) - was spreading from continent to continent unhindered. Antibiotics appeared to be useless in the battle to contain the sickness, which was merrily winging its way on board the hundreds of international airliners criss-crossing the planet every hour.

It was a terrifying prospect, made no less alarming by the WHO's subsequent emergency travel advisory notice urging travellers to see their doctor if they developed symptoms, which include a high fever (38.6C), muscle aches, headache, sore throat, dry cough and shortness of breath.

Last night the deadly disease seemed to have landed in the UK. A man, who had travelled from Hong Kong to Manchester via Amsterdam on Saturday, was being held in isolation at the specialist infectious diseases unit at North Manchester General Hospital. According to Christine McNab, spokeswoman at the WHO, suspected cases, yet to be confirmed, were cropping up all over the world.

Panic is setting in. In Hong Kong, one of the key sites of the disease, everyone from taxi drivers to tourists is wearing masks in a desperate attempt to avoid infection. Passengers boarding aeroplanes are discreetly examined for symptoms and turned away if there are any suspicions. Some countries are spraying aircraft interiors with disinfectant and antiviral treatments to try to combat the disease.

Like ebola, CJD and Aids, SARS is frightening because it is incurable. So far, laboratories in five countries have failed to detect a cause. The prime minister of New Zealand, Helen Clark, raised panic levels even higher when she warned that SARS could be as deadly as the 1918 flu pandemic that killed up to 50 million people around the world.

But how scared should we be? If you work in a big office, where international travel is not uncommon, how many of your colleagues may have been on board a flight carrying someone affected by the disease? Should we be cancelling trips to such areas of the world as Hong Kong, China and Vietnam, which have been worst hit by the illness? And how serious is it if you get it?

So far, nine people have died from the condition, which is believed to be linked to an earlier outbreak in China last November. Many others, however, are critically ill with a severe, atypical pneumonia, and are breathing through respirators.

McNab admits that it is rare for the WHO to issue such alerts. "There is a certain level of alarm out there," she says. "People are calling us wanting to know if they should travel. But to put it in perspective, we are looking at 500 cases globally on this, largely confined to three sites - Hong Kong, Vietnam and China. Of those countries, 90% of the cases have been in a hospital setting, with nurses and doctors treating those affected.

"The rest of the cases seem to be people in very close contact with affected people. That narrows the risk to the general population. The pattern of how this is moving does not indicate at this point that there is a widespread risk to the general population. The reason we issued the alert is because we have not been able to determine exactly what it is. What we want is to make travellers and airlines aware of the symptoms of this.

"The alert has greatly helped. We are getting a good picture overall of what's going on globally. Surveillance is much more sensitive because people are aware. People should not panic. We have not issued any kind of travel restrictions. The number of cases is still relatively low. It's premature to equate it to any previous outbreaks or pandemics."

The New Zealand prime minister's warning may have been unnecessarily alarmist, but the WHO is in an ongoing state of high alert because of fears about a new flu pandemic. Public-health authorities around the world have been bracing themselves for a new strain of influenza, which is among the most infectious and deadly diseases known to man.

"Influenza is the big number-one virus that would spread around the world and cause mayhem," says John Oxford, professor of virology at the Queen Mary School of Medicine in London. "Influenza is very aggressive with a considerable punch. That's why we're all on tenterhooks. There hasn't been an outbreak for a while and we're expecting another one.

"As far as I can see at the moment, they've excluded influenza. No one is talking about influenza, so you move down the shopping list of micro-organisms, none of which is quite as bad as influenza. One is concerned about SARS, but not alarmed. There are no alarm bells ringing in my head."

But yesterday, David Heymann, head of communicable diseases at the WHO, appeared not to rule out influenza. "The reason we are alarmed is because we do not know what is causing it. If this is a disease like a new influenza, it could spread very rapidly throughout the world."

There have been three certified flu pandemics. Apart from the 1918-19 catastrophe, when the only country in the world untouched was Samoa, a further five million died in an outbreak in 1957, and a further two million died in 1967.

One of the key characteristics of the flu virus is the ease with which it spreads. "There are few organisms which are so infectious as influenza," says Oxford. "It has the potential to spread like wildfire."

Although the disease has already travelled far in this particular outbreak, its progress is more limited than might be expected of a typical flu pandemic. As well as the three main sites and the suspected British case, there have been cases reported in Switzerland, Germany, Taiwan, Canada, Singapore, Thailand and the Philippines.

"It does not look as though it's explosively infectious," says Oxford. "Most of the cases seem to have come out of hospitals, among doctors and nurses, all of whom have been in very close contact with ill people. It is causing problems in certain environments, but it's not zipping around the globe."

The one factor worrying him, he says, is the age of people dying from the disease. Usually it is the elderly who are vulnerable to such illnesses, but in this case apparently healthy working people are falling victim and dying. X-rays of victims' lungs show pneumonia or respiratory-distress disorder; lab tests show low numbers of white blood cells and platelets.

"The reassuring thing is that it does not appear to be influenza A," says Oxford. "That makes me feel more relaxed. If it was influenza A, we would be dusting down our pandemic plans. One day, one awful day, those plans will have to be brought out."

The last time the plans were dusted down was in 1997, when a particularly virulent H5NI strain of the flu virus surfaced. A three-year-old boy was the first of several to die after playing with chickens at his nursery. It was discovered that the virus had jumped from chickens to humans - which caused widespread alarm - but that particular strain was not transmissible between humans.

The WHO is hoping that a breakthrough against SARS will be found by studying the progress of the disease in China, which first saw cases in the Guangdong district last November, though it has not been confirmed as the same virus. "With four months of meticulously obtained information from China, we believe we will have the answers to how this disease acts in populations and how we can best deal with it," says Heymann. Such outbreaks often seem to originate in China and the surrounding region, according to Oxford, simply because of the density of the population.

So how worried should we be? "You should not be worried at all," he says reassuringly. "I'm certainly not. There may be a case in England, but you've got more chance of stepping outside your office and being run down by a taxi at the moment.

"It's difficult to assess the risk in the future. You just have to keep your eye on it."