Johannes (Hans) Sauer, medical practitioner and businessman, was the son of Johannes J. Sauer, landdrost of Smithfield, and his wife Elizabeth W.S.M. Kotzé. Hans junior grew up in Aliwal North and Burghersdorp. He left South Africa for Britain in 1876 and qualified as Bachelor of Medicine (MB) and Master of Surgery (CM) in Edinburgh in 1881. Upon his return to the Cape he was licensed to practice in April 1882 and settled in Kimberley. In May that year a minor epidemic of smallpox broke out in the Cape Peninsula. The financier Cecil J. Rhodes and his associates in Kimberley were afraid that the disease would spread to the diamond fields and cause labourers to flee the area. To avoid the resulting financial loss a quarantine depot was established on the main road at a drift in the Modder River, some 50 km south of Kimberley, where Sauer was stationed in September 1882. Without any legal authority he closed the other routes to Kimberley from the south and demanded proof of vaccination against smallpox from all travellers, had them fumigated with burning sulpher, and put suspect cases in quarantine. The operation, during which he found 14 cases of smallpox, continued for more than twelve months. Many cases of illegal detention and assault were brought against Sauer and his assistants, but Rhodes and the mining houses smoothed matters over and bore the cost.
In October 1883 a small group of labourers from Mozambique arrived ill near Kimberley. The civil commissioner of the town sent a team of six or seven medical practitioners to investigate the disease. Although they appear to have initially diagnosed smallpox, several of them soon afterwards claimed publicly that it was pemphigus (a rare skin condition). On account of Sauer's previous experience at the Modder River he was recalled by the government of the Cape Colony from a visit to the eastern Transvaal (now Mpumalanga) and asked to look into the matter. He satisfied himself that the disease was indeed smallpox and proceeded to Cape Town, where he was appointed medical officer of health for Kimberley. Upon his return the disease had reached epidemic proportions. A vaccination campaign was launched, but the mining community refused to cooperate. A number of doctors, including Rhodes's associate Dr L.S. Jameson, continued to refuse to diagnose or report cases of smallpox and the resulting split in the medical community became known as the "smallpox war". Sauer requested help from his brother, a prominent Cape politician, and members of the government, and as a result the Public Health Act of 1883 was passed by the Cape legislature, making vaccination and the notification of infectious diseases compulsory and granting extensive emergency powers to local authorities. Promulgation of the Act made Sauer unpopular, but enabled the local authority to bring the epidemic under control by the end of 1884. The diagnosis of smallpox was confirmed by an independent medical officer from Cape Town.
As early as 1883 Sauer had been admitted to practice medicine in the South African Republic. In 1886 he visited the Witwatersrand to inspect the earliest discoveries of gold there. After showing some ore samples to Rhodes the latter sent him back to the gold fields to buy claims, offering him a share of the profits. By the end of 1886 most of the gold-bearing properties had been bought by big financiers, with the result that Sauer ended his agreement with Rhodes's syndicate and was payed out in shares. Meanwhile in November 1886 he had been elected a member of the Diggers' Committee, which was charged with overseeing the orderly development of Johannesburg. Effective sanitary arrangements were a priority and at the Committee's first meeting Sauer and two other members formed a sub-committee to report on this matter. In March 1887 he was appointed as the first district surgeon for the Witwatersrand. He also became chairman of the Sanitary Committee and chairman of the Rand Club, while Sauer Street in Johannesburg was named after him. He was a well-known figure in Johannesburg, partly because of his involvement in professional and social exploits that several times ended in the law courts. Thickset, jovial and adventure-loving, he was a gambler and happiest in dangerous situations.
In 1890 Sauer married Cecile J. Fitzpatrick, sister of Sir Percy Fitzpatrick. That same year he resigned as district surgeon, gave up medicine, and proceeded to London to study law. However, early in 1891 Rhodes persuaded him to suspend his studies and go to present Zimbabwe to investigate prospects there for the Zambesia Exploration Syndicate. After an extensive tour through the territory Sauer returned to London, sold his interests in the syndicate, and continued his studies. However, in May 1893, just before his final examination, Rhodes offered him a post as manager of the Rhodesian Exploration Syndicate, a post he held until 1910.
Towards the end of 1895 tension between the English speaking community on the Witwatersrand and the Afrikaner government of the South African Republic reached breaking point. Sauer's sympathy lay with the English immigrants, which led him to join the Reform Committee in Johannesburg just before the ill-fated Jameson raid, aimed at overthrowing the government. He was arrested for treason early in January 1896 and sentenced to two years in prison, but was released on payment of a fine. Upon his return to Zimbabwe he participated in quelling the Matabele Rebellion of 1896 and the subsequent political negotiations, and then assisted Rhodes in England during a parliamentary enquiry into the Jameson raid. In 1899 he became a foundation member and the first president (for 1899/1900) of the Rhodesia Scientific Association in Bulawayo. He served as president again in 1901/2. During this time he read a paper on "Malaria" at a meeting in September 1901. He was still a member of the association in 1918. Meanwhile he had settled in England in 1900, returning to Zimbabwe for a few months each year. He even served as a member of the territory's Legislative Council.
Sauer retired in 1910, after which he spent much time on the European continent. His autobiography, Ex Africa (London, 1937) contains a fascinating account of his adventurous life in southern Africa during the latter part of the nineteenth century.