Wired on Friday: Severe Acute Respiratory Syndrome (SARS) is now a worldwide epidemic, in part because of the Chinese government's reluctance to share data during the early stages of the disease.

If the World Health Organisation had information earlier, it might have been able to put procedures in place to halt the spread of the disease.

To address issues such as these, a monitoring system has been in place in New York City for a number of years and was given added impetus in the wake of September 11th, 2001, by the growing fears of a bioterrorism attack.

New York City's Syndromic Surveillance System identifies trends and patterns in health-related areas citywide. Changes or abrupt spikes in these patterns may indicate the emergence of a new or serious illness.

It is a computer software system that the city developed in 1994 and is considered a model for other cities.

In 1993, a huge water-born parasite called cryptosporidios contaminated the water supply in Milwaukee. About 400,000 people fell ill, 4,000 were hospitalised and 100 died.

Milwaukee's Department of Health officials did not recognise people's symptoms quickly enough and afterwards realised they should have spotted early indications of the illness, such as the vast number of people buying anti-diarrhoea medications or visiting hospital emergency rooms.

If the Milwaukee Department of Health had had a surveillance system linked to the hospitals and the pharmacies, it may have picked up on the disease before it killed so many people.

New York City has the largest unfiltered water system in the US and the Environmental Protection Agency put pressure on the city government to learn lessons from Milwaukee's water disease experience.

So city officials established two systems: one to look at pharmacy sales of anti-diarrhoea tablets and a second to check the stool submissions that doctors were requesting to be sent to laboratories.

In 1998, the system was broadened to monitor ambulance transportation to hospitals. After the World Trade Centre and anthrax attacks in 2001 the health department "jumped into surveillance with a vengeance", said Mr Richard Heffernan, director of the data analysis unit at the Bureau of Communicable Diseases, which is part of the city's Department of Health.

It began to monitor emergency room visits.

Setting up the Syndromic Surveillance System was initially expensive and New York City's health department estimates that it spends $500,000 (€454,000) a year to operate it, mainly in labour and computer expenses.

The expanded system includes several databases that look at emergency room visits, pharmacy sales, ambulance transports, 911 calls and worker absenteeism.

Across the five boroughs of New York, there are 68 emergency departments. Forty of these, which account for 74 per cent of emergency visits to the city's hospitals, participate in the Syndromic Surveillance System.

According to Mr Heffernan, each year these systems give the earliest indications of community-wide influenza.

For example, they noted patterns in the flu illness in New York during November and December 2001 and 2002. They spotted a short spike in a gastrointestinal illness during last autumn. They also recognised that the strain that caused the flu outbreak in the city, which featured vomiting and diarrhoea, was the same one that grounded several cruise ships because of illness on board.

The systems can pick up early signals from the data that reach central servers at the Department of Health and Mental Hygiene's headquarters in lower Manhattan.

The 40 participating hospitals electronically transmit data every morning.

In addition, a large pharmacy retail chain provides data files daily of all its sales the previous day.

These data, comprising 50,000 pieces of information daily, are combined and the Bureau of Communicable Diseases starts its analysis. The analysis can take two to three hours, Mr Heffernan said.

When the research scientists spotted the flu outbreak last year, they sent an e-mail and fax to all the physicians in the city. They contacted all the city's hospitals and asked the doctors there to increase testing.

"Basically we tried to define the cause of the illness," Mr Heffernan said.

Although Boston, Los Angeles and Washington have similar efforts under way, none is as developed as New York's.

It is thought the systems could also be beneficial in reducing the impact of bioterrorism attacks through earlier detection and action than is possible with traditional diagnosis-based surveillance.

According to Mr Heffernan, "this whole field is very new but is something of public health value".

Ten years from now, he expects it will move from being experimental to becoming a permanent part of the public health system in the same way the surveillance of salmonella became a permanent part of the system a decade ago.

With the use of technology, "we're harvesting data and making secondary use of data that is already out there in the system," he added.