HONG KONG: Ebola, SARS, avian flu, mad cow disease: We live in the shadow of new epidemics. The avian virus, for instance, might mutate, we are told, learning to transmit itself directly from human to human. But is that possibility 50:50, 50:1 or 500:1?

No sensible policy is possible without assessing risks and the likely costs of avoiding them. It may be the World Health Organization's duty to warn of dangers, but societies have broader interests in weighing costs and risks and a right to subject current scientific wisdom to scrutiny.

Experts fret that globalization is increasing the risk of the spread of new diseases. In response, societies are being urged to arm themselves with ever greater precautions. At the official level there has been culling of animals and quarantining . Unofficially, there has been a more dramatic response. Travel collapsed last year because of fear of severe acute respiratory syndrome, or SARS. Countries as far apart as Japan and Australia were badly hit even though they had no cases of SARS, whose method of propagation is still little understood. Now, avian flu threatens a repeat.

But take a look at the history of alarms and responses. Back in 1851, nervous experts worried that the Great Exhibition of that year could attract so many travelers to London that new diseases might be brought to Britain. But professional worriers and self-proclaimed experts did not always have things their way.

I have a paper delivered by a forebear, John Bowring, then a prominent liberal political economist and later governor of Hong Kong, published by the British Association of Science in 1838 entitled “Observations on the Oriental Plague and on Quarantines as a means of arresting its progress." The author was not a doctor but he did understand statistics and the processes of government. He studied the issues firsthand in Turkey and Egypt.

SARS caused less than 1,000 deaths worldwide. About 200,000 died of the plague epidemic that hit Egypt in 1834-1835. Lesser outbreaks occurred spasmodically throughout the Levant. The official response was usually draconian quarantine rules by which whole cities, such as Alexandria, were surrounded by military cordons. Officials were given exceptional powers to limit movements.

The contagiousness of the plague was assumed to justify quarantines but Bowring concluded that they were largely useless. Officials failed to take into account that plague was endemic. A Scottish doctor, long resident in Alexandria, ascribed the zeal for quarantines as due to “superstition and ignorance."

There was no evidence that quarantines affected the spread of the disease. Major plague outbreaks - like SARS? - were associated, Bowring said, with particular weather conditions and were specific to the area where they prevailed. Death rates were largely a function of overall living and sanitary standards - overcrowding and poor ventilation being the main determinants. Quarantine of humans could not stop spread of the virus by birds and animals. (The WHO now says that wild birds can spread avian flu across continents.)

Quarantines were anyway impossible to enforce effectively, Bowring wrote, because of the inefficiency and corruption of government employees - a sentiment that could be echoed in China and elsewhere today. And, he asked: “Will the adventurous Khurd, the migratory Turkoman, the fanatical Pilgrim, the potent Sheikh, be stopped in their peregrinations by the quarantine?"

Bowring concluded that the net result of the quarantine system was not to improve health security but to submit travelers to “capricious and despotic” actions by officials. They inflicted huge losses on commerce and added to “the miseries which it is their avowed object to modify or to remove." Support for the quarantine system was partly due to ignorance but partly to vested interests. Rumors and unsubstantiated claims “were for the most part from persons connected with boards of health or quarantine establishments having a pecuniary interest in the question.”

These 1838 remarks might cause us to ask whether in the age of SARS and bird flu alarms it is not time to subject the statements of certain virologists, headline writers and health bureaucracies to critical analysis by those trained in other disciplines.