One of the symptoms of the bubonic plague is a nasty-smelling rash on the skin. In the 17th century, people carried an assortment of fragrant herbs to mask the smell. (The rash is the ‘ring o’ roses’ and the herbs are the ‘pocketful of posies’ in the nursery rhyme.)
As medicine has advanced, understanding smell rather than masking it has become an area of research. Gas sensors can test the components in someone’s breath to indicate some diseases.

In 2018, Fujitsu Laboratories developed a portable breath sensor as an alternative to large analytic instruments used to detect high blood pressure, diabetes and cancer. The breath sensor analyses gas components which enter the breath from the body’s organs, and which evaporate in the lungs to be exhaled in the breath. Specific gas components can be an indication of some health conditions. For example, ammonia concentration is correlated with liver metabolism and the infection Helicobacter pylori. Combined, these...