There has been a lot of talk lately about global contagion. With the banking crisis the talk of an epidemic is a metaphor. The risk with the swine flu scare is that it turns into a genuine pandemic.

There have already been 100 deaths in Mexico and the Health Secretary, José Ángel Córdova, has admitted to more than 1,600 suspected cases. A public health emergency has been declared in the United States, where 40 cases have been confirmed and 17 more are suspected in a New York school. There are six confirmed cases in Canada and one in Spain. Tests are also under way on suspected outbreaks in New Zealand, Australia, Brazil, Britain and Israel.

It is too early to be sure that this is a pandemic in the making but not too early to say, as indeed the United Nations has done, that it could be. This inevitably throws up the spectre of the 1918 Spanish flu pandemic, the worst of modern times. Forty per cent of the world's population were infected and 50 million people were killed. In 1957 the Asian flu virus, mutating from a strain found in wild ducks, killed two million people. A flu outbreak in 1968 in Hong Kong killed up to one million people globally.

So this is no idle threat. The pertinent question, then, is how prepared are governments in the face of the threat? It can be very hard to gauge the level of threat correctly and responses can have unintended consequences. After 9/11 many Americans decided to drive rather than fly. The shift lasted for a single year before traffic patterns returned to normal. A psychologist called Gerd Gigerenzer has calculated that more than 1,500 Americans died in fatalities on the road who would ordinarily have been flying, safely.

The initial responses of the multinational arms of government were as prompt as one could expect of such cumbersome bodies and reassuring as a result. The World Health Organisation (WHO) met in Geneva yesterday and the Director-General has said the threat constitutes a public health emergency of international concern. The European Commission is calling an urgent meeting of health ministers to discuss the situation. The World Bank is providing Mexico with more than $200 million in loans to help it to deal with the outbreak.

National governments have reacted in different ways. Russia, China and Thailand have banned the import of pork products from Mexico and affected parts of the United States, although there is no evidence, as yet, to link infection with exposure to pork. A number of countries in Asia, Latin America and Europe have begun screening airport passengers for symptoms.

In this country, government spokesmen have followed the rubric that is now common: speak regularly and be reassuring but candid. The chief medical officer, Sir Liam Donaldson, warned that the virus would arrive on these shores, as it now has. Mr Donaldson has confidently said that planning has been going on for five years, in preparation for bird flu.

The Health Secretary, Alan Johnson, has said that the NHS has a stockpile of more than £500 million worth of the Tamiflu antiviral drug which, in the absence of a vaccine, is the best that science can do. Mr Johnson also pointed out that existing EU rules prevent the import of all live pigs and pig meat from Mexico.

A test on a Canadian woman in Greater Manchester turned out to be negative. But two cases have already been confirmed in Airdrie in Scotland. Public concern is natural. For the moment, we can do no better than follow the rather homely advice of the Health Protection Agency: people should cover their nose and mouth when coughing or sneezing, throw away dirty tissues promptly and wash hands and surfaces that are regularly touched.