John Pratt-Johnson (sometimes John P. Johnson), medical practitioner, qualified as Bachelor of Medicine (MB) and Bachelor in Surgery (BS) at the University of London, and as a member of the Royal College of Surgeons (MRCS) of England, and a licentiate of the Royal College of Physicians (LRCP) of London, all in 1907. In 1912 he furthermore obtained the Diploma in Public Health (DPH) at the University of Oxford. Meanwhile he had been appointed around 1910 as assistant bacteriologist of the Transvaal, a post he appears to have held to 1912. By 1913 he was acting medical officer of health of Johannesburg and that year compiled a report, On the circumstances of milk production in and around Johannesburg (1913, 35p). That same year he was registered to practice medicine in the Transvaal.
During World War 1 (1914-1918) Pratt-Johnson served in the South African Medical Corps as officer commanding the First Field Ambulance, with the rank of Major. By 1917 he was director of the Clinical Research ('Clinsearch') Laboratories in Johannesburg, which prepared vaccines and sera, and rendered pathological and bacteriological services. That year he published a Manual of the Clinical Research Laboratories, Johannesburg (1917, 53p). Some of his work was connected with the mining industry, as indicated by his paper on 'Silicosis in rats in Witwatersrand mines' (Journal of the Chemical, Metallurgical and Mining Society of South Africa, March 1917). At this time he was still associated with the South African Medical Corps, as deputy assistant director of medical services (pathology). During 1917-1919 he was involved in research on malaria at the Military Hospital in Pretoria, where malaria patients were sent from the military campaign in German East Africa. In collaboration with K. Gilchrist and Capt. Hay-Michel he published a paper, 'On the action of certain special preparations on malarial parasites and their employment in the treatment of malaria' (British Medical Journal, 1921). He and Gilchrist also reported on 'A statistical investigation of records of treatment and blood examination in 18 731 cases of malaria' (The Lancet, 1921).
After the war Pratt-Johnson published a report on Epidemic influenza and inoculation (Johannesburg, c. 1920, 38p) in which he pointed out the value of inoculation, with special reference to the influenza epidemic of 1918. During the next decade he produced some more publications, including the following: 'Bacteriology of pyorrhoea' (Medical Journal of South Africa, 1924/5); and 'New methods in the prevention of diphtheria' (South African Journal of Science, 1928).
Pratt-Johnson was married to Jean Margaret Julia Ashburnham Pratt-Johnson, with whom he had two surviving daughters and a son.