Earlier this month Web company Google demonstrated how it could mine its own search tracking database to track the spread of influenza across the US. By analysing certain key search terms such as 'flu symptoms', or 'flu medicine' made within a certain geography, Google is able to track the incidence and spread of flu within a region. Although a search for flu-related symptoms doesn't necessarily mean that the searcher has influenza, there is a remarkable correlation to real illness data.

Google compared its historical search data over five years with actual data on flu incidence collected by the US Centers for Disease Control and Prevention (CDC) via a network of doctors and other health professionals and the search graphs matched CDC's almost perfectly. The advantage of the Google approach is that it can give health authorities early warning of a possible flu outbreak so that authorities can take appropriate action. The CDC method takes much longer to collate the data, by which time it could be already too late.

While this system can help track normal incidence of winter flu, there is some doubt whether it can help much with epidemic outbreaks such as the Asian Avian flu pandemic. In these cases the media furore surrounding the outbreak will probably have everyone turning into a web hypochondriac.

But this type of data mining raises some pretty serious privacy issues, says analyst Ovum. Most people would agree that fighting influenza pandemics is something in the public interest, the concern Ovum raises is how far this could go, particularly if the data ends up being attributable to an individual? Does looking up search terms really serve as evidence that an individual has influenza. Would they perhaps be denied the right to travel? Or even more ominously, the same technology could be used to identify other public health issues – would looking for AIDS symptoms, for example, mark an individual a possible AIDS carrier in their digital footprint.

Google, of course, is aware of these privacy issues, and says, "Google Flu Trends can never be used to identify individual users because we rely on anonymized, aggregated counts of how often certain search queries occur each week." But this could easily change if there was a flu pandemic, which is one of the greatest threats facing most countries today. If there was an outbreak then authorities would do everything in their power to stop the spread of the disease. This could also include Google search terms as already happens in the fight against terror. Once that genie is out of the bottle, it will be impossible to put it back.