Chillies belong to the Capsicum genus in the nightshade family.
There are five domesticated species, but most of the cultivars we use in cooking have been bred from Capsicum annuum.
Only mammals are susceptible to the chemical compounds, known as capsaicinoids, in a chilli.
These are concentrated in the membrane surrounding the seeds (not in the seeds, as is often thought).
The level of these chemicals varies depending on the variety of chilli and how it is grown.
Wild chillies have evolved to attract birds, which are completely unaffected by capsaicinoids.
Studies have found that birds not only disperse the seeds, but also improve them.
Scientists at the University of Washington discovered that the seeds of Capsicum chacoense were cleansed of pathogens such as fungi as they passed through the avian digestive system, making them less attractive to predators and increasing their chances of survival by 370 per cent.