Some infectious diseases also give off characteristic smells. Sweet-smelling poo could be a sign of infection with cholera or the Clostridioides difficile bacteria, which is a common cause of diarrhoea – although one study found a group of unfortunate hospital nurses were unable to accurately diagnose patients by sniffing their faeces. Tuberculosis, meanwhile, can cause a person’s breath to smell foul, like stale beer, and their skin like wet brown cardboard and brine.
Detecting other diseases, however, requires a special kind of nose.
Dogs, for example, have a sense of smell that is reportedly up to 100,000 times stronger than ours. Scientists have trained canines to sniff out lung, breast, ovarian, bladder, and prostate cancers in people. In one study on prostate cancer, for example, dogs were able to detect the disease in urine samples with a 99% success rate. Dogs have also been trained to detect early signs of Parkinson’s disease, diabetes, oncoming epileptic seizures, and malaria, all from smell alone.
But not all dogs have what it takes to become a disease detector, and it takes time to train the animals that do. Some scientists say we can replicate the amazing olfactory capabilities of canines, and people like Milne, in the laboratory, perhaps offer the chance to have a simple swab that could be sent off for testing.
Barran, for example, is using gas chromatography-mass spectrometry to analyse sebum (an oily substance produced on people’s skin) from Parkinson’s patients. Gas chromatography separates the compounds, and mass spectrometry weighs them, allowing you to determine the precise nature of the molecules present. The food, drink and perfume industries already use this form of odour analysis routinely.