For the past few years, I have been developing methods to exploit sophisticated machines to identify chemicals that may diagnose disease. The machines, called mass spectrometers, weigh individual molecules. We can quantify hundreds of chemicals from blood, urine or even breath, seeking changes in chemical balance associated with diseases.

Catching diseases before symptoms appear improves the chances of curing them. Some biological sensors are even more sensitive. In August, a group in Germany showed that dogs can be trained to "smell" lung cancer by recognising changes to volatile organic compounds in the breath. Olfactory receptors in dogs' noses bind to specific chemicals and their brain then processes the signals, allowing them to smell the cancer.

Other animals can be used, too. Apopo, a company based in Tanzania, trains African giant pouched rats to sniff out landmines. Rats can also detect tuberculosis from the breath of infected patients, allowing faster and cheaper diagnosis than laboratory tests. One problem with animal sensors, however, is the biology. What happens if, for instance, a sick rat loses precision? And not all diseases provide compounds that noses can detect.

As good as the animal approaches are, we'll keep seeking machine-based approaches to sniffing for disease.