Within two years, saliva tests could warn of oral cancer, respiratory diseases, anthrax infection and even dental cavities in very young children.

Spit tests and mouth swabs already provide reliable evidence of blood alcohol, steroid abuse and narcotics.

But the combination of genetic discovery, sophisticated micro-circuitry and advanced medical science could result in a new generation of detectors, little bigger than a fountain pen, which could provide early warning of a range of diseases.

Humans produce a litre of saliva a day on average. A tiny smear contains much the same information as a sample of urine or blood.

Paul Denny, of the University of Southern California's school of dentistry, told the American Association for the Advancement of Science conference that a simple saliva test could predict whether children would develop cavities, how many they might develop and even which teeth might be the most vulnerable. One test measured the relative proportions of sugar chains in the mouth and on tooth surfaces. One kind of sugar chain repelled decay bacteria, another kind allowed bacteria to bond to a tooth and start the decay process. The proportions of the sugar chains were determined genetically and could not be changed.

Over the years researchers were able to match sugar chain proportions with the number of cavities in young adults.

So the evidence in the saliva meant dentists could predict a child's future dental history with 98% confidence, and suggest a course of preventive action.

Daniel Malamud, of the University of Pennsylvania, told the meeting he was working on a saliva test for HIV and a bacillus closely related to anthrax. "Nearly everything that is going on in your body reveals itself in some way in the fluids in your mouth - whether it is from saliva, mucus or the plaque on your teeth," he said.