Twort, a doctor's son from Camberley in Surrey, qualified in medicine in 1900 and after various appointments in London hospitals became professor of bacteriology at London University. His most important discovery was made during an attempt to grow viruses in artificial media: he noticed that bacteria, which were infecting his plates, became transparent. This phenomenon proved to be contagious and was the first demonstration of the existence of bacteria-infecting viruses. These were later called ‘bacteriophages’ by the Canadian bacteriologist Felix d'Herelle, who discovered them independently.

Twort was also the first to culture the causative organism of Johne's disease, an important intestinal infection of cattle.