Downie, Allan Watt (1901–1988), medical microbiologist, was born on 5 September 1901 in Rosehearty, Aberdeenshire, the fifth child in the family of seven sons and one daughter of William Downie, a deep-sea fisherman, and his wife, Margaret Watt, daughter of a fisherman from Fife. The younger of identical twins, he and his twin, Ricky Downie (1901–1978), grew up close to Rosehearty harbour and became familiar with the sea. They were educated at Rosehearty School, where their unusual ability was spotted, and at Fraserburgh Academy. In 1918 they entered Aberdeen University medical school, from which in 1923 they graduated MB ChB with first-class honours and the distinction of collecting between them every subject prize in every year of the course. In 1923 Allan Downie was in general medical practice in Sheffield and from 1924 to 1926 was a lecturer in bacteriology at Aberdeen University. He obtained his MD in 1929 and DSc in 1938. In 1927 he moved to the department of pathology in Manchester University, where he turned to the new science of virology. With a veterinary pathologist he for the first time demonstrated, in tissue culture, the cellular changes which characterized in its natural animal host, the disease mousepox, a model for human smallpox. This little-noted paper opened a new chapter in methods for studying viruses and virus diseases.

In 1935 Downie married Annie (Nancy), schoolteacher, daughter of William Alan McHardy, wood engineer; they had two daughters and a son. That year he also won the senior Freedom research fellowship at the London Hospital medical school. First, however, he had to spend a nine-month academic year at the Rockefeller Institute in New York City under O. T. Avery and alongside the future leaders of American microbiology. At the London Hospital, Downie initiated work on pox viruses and defined for the first time the distinction between vaccinia and cowpox viruses. This later led him on to smallpox and to its ultimate eradication. The outbreak of the Second World War stalled his work on pox viruses, when he was directed to head the Emergency Public Health Laboratory Service in Cambridge, one of the regional laboratories providing expertise in public health for disease control, water-supply monitoring, and possibly bacterial warfare. (The east coast was a probable front line should invasion happen.) Downie was at Cambridge until 1943, when he was appointed professor of bacteriology in Liverpool.

Returning troops and the resumption of foreign trade after the war brought numerous imports of smallpox into Britain. Downie's laboratory in Liverpool became the world centre for the study of smallpox: of how the virus entered its victims, spread inside them, and then passed to others; of precisely when the patient became infectious and for how long. These studies progressed for twenty-two years, and then the World Health Organization recognized that an effective smallpox eradication plan was possible. With Downie's guidance the intensified and successful programme was launched in 1966, the year of his retirement. Since 1978 there has been no smallpox case; there are no human carriers and no animal cases or carriers. The disease which in the 1960s was killing ten million people per year has ceased to exist. Many thousands of public health workers took part in the programme and the credit, as Downie would have wished, has been spread worldwide. None can doubt that in the laboratory in Liverpool, in the field in India, and after his retirement at the World Health Organization at Geneva and in training courses in Denver, Colorado, Downie's contribution was paramount. It was the greatest medical triumph of the century. Downie helped to train more than 3000 doctors and published 110 outstanding papers. A founder fellow of the Royal College of Pathologists, he was made an honorary LLD at Aberdeen University (1957). He became a fellow of the Royal Society in 1955 and an FRCP in 1982.

Short and wiry, Downie had a great affection for sport. In Manchester he played left-half for Whalley Range Football Club in the Lancashire amateur league. He spent every summer holiday in Rosehearty, with Ricky and his family, sailing in the heavy old family sailboat, with grandchildren or friends, and happy to be on the water and under sail. He loved to fish and to identify sea birds, but it was at golf on the Royal Birkdale course, near Southport, that he excelled and was never satisfied, striving always to reduce his (most enviable) handicap. When he retired from his Liverpool chair in 1966, the Southport Visiter heralded the news with the headline 'Noted Local Golfer Retires'. He was a smoker, as was his twin, Ricky; both died of lung cancer. Allan Downie died on 26 January 1988 in Southport, Lancashire, and was cremated there.