Boyd, Sir John Smith Knox (1891–1981), bacteriologist, was born on 18 September 1891 at Largs, Ayrshire, the second of three sons (there were no daughters) of John Knox Boyd, an agent in the Royal Bank of Scotland, and his wife, Margaret Wilson Smith. He was educated at the local school in Largs and then entered Glasgow University in 1908 to study medicine; he graduated MB, ChB in 1913. After house appointments at Glasgow Royal Infirmary he sailed to Rangoon as a ship's surgeon. On his return in 1914 Boyd applied for a commission in the Royal Army Medical Corps. By December he was at Ypres. From France he moved to Salonika, and became medical officer to the divisional engineers in 1916. In this post he travelled widely through Mesopotamia, making his first acquaintance with tropical diseases. After some training in bacteriology, by 1917 he was in charge of a mobile laboratory where he worked on the treatment of malaria and studied the prevalent dysentery. He became a pathologist at Salonika in September 1918 but was invalided home in December with ‘Spanish influenza’. In the same year he married Elizabeth (d. 1956), daughter of John Edgar, a Dumfriesshire station-master.

Having obtained a regular commission in the Royal Army Medical Corps, in 1920 Boyd was appointed to head the brigade laboratory at Nasirabad in Rajputana and later the district laboratory in Mhow, Central Provinces. He returned to London in 1923 and joined the staff of the Royal Army Medical College, where he became demonstrator and then assistant professor of pathology. He obtained the DPH at Cambridge in 1924. Back in India in 1929 with the rank of major he was given charge of laboratories in Bangalore and then Poona. In 1932 he was appointed assistant director of hygiene and pathology at the army headquarters, Simla. In 1936 he returned to Millbank to run the vaccine laboratory. He was awarded the Leishman medal in 1937 and promoted to lieutenant-colonel in 1938.

Boyd put the vaccine department of the army medical college on a war footing in 1939 and after its transfer to Tidworth it produced sufficient material for all the needs of the services during the Second World War. He organized a blood transfusion service for the Middle East forces and pioneered the preferential use of whole blood for transfusion in casualties with severe blood loss. During this time he built up some forty laboratories in that zone. He was mentioned in dispatches (1941). In August 1940 he was also given responsibility for pathology in the Middle East and in November 1943 he became deputy director of pathology to the twenty-first army group. Promoted colonel in 1944 Boyd became a brigadier in the following year. In 1945 he became director of pathology for the War Office.

Boyd left the army medical staff in 1946 and became director of the Wellcome Laboratories of Tropical Medicine where he remained until 1955. In that year he became a Wellcome trustee and served until 1966; he also became deputy chairman in 1965 and a consultant to the trust until 1968. Boyd's first wife died in 1956 and in 1957 he married his secretary, (Ellen) Mary Harvey Bennett (1910–1968), daughter of Denis Harvey Murphy, company director, of Northwood, Middlesex, and Mary Ellen Dempsey. There were no children of either marriage.

Boyd was especially interested in the dysenteric diseases. He studied the difference between ‘smooth’ and ‘rough’ colonies of dysenteric bacterial strains, showing that the rough strain contained the group antigen common to all flexner types while the smooth strain lacked this group antigen but possessed its own specific surface antigen. This property made it possible to separate dysenteric bacilli into two groups subsequently called flexneri and boydii. Later, in the Middle East, Boyd was responsible for the first trials of sulphaguanidine in the treatment of dysentery. His work on malaria included the first studies on synthetic anti-malarial preparations, and his studies of typhus in India in 1916 showed that most cases of this disease were transmitted by the mite and the flea, and not by the tick as had been suspected. Boyd also had a special interest in bacteriophage. His experience with these ‘bacterial viruses’ led him to formulate a theory for the long-lasting immunity that occurs after recovery from yellow fever, based on the bacteriophage model.

Boyd was a member or chairman of many committees and was president of the Royal Society of Tropical Medicine and Hygiene (1957–9). He was awarded the Manson medal in 1968 and was elected MRCP in 1950, and FRCP and FRS in 1951. He graduated MD from Glasgow in 1948. He was an honorary FRCPE (1960) and FRSM (1965). He was made FRCPath (1968), honorary LLD, Glasgow (1957), and honorary DSc, Salford (1969). He was appointed OBE in 1942 and knighted in 1958.

Boyd was a deeply honest man of military bearing with a determination to see that his objectives were achieved. During his latter years he was a very formidable figure, but once he made a friend his loyalty was unshakeable. He was a keen golfer and bird-watcher. Boyd died on 10 June 1981 at Northwood, Middlesex.