Walter Frei qualified as a veterinarian at Berne Veterinary College in Switzerland. In 1906 he was awarded the degree Dr. Vet. Med. by the University of Zurich with a thesis titled Zur Theorie der Haemolyse.... He then worked as an assistant in the Physiological Laboratory of the Poppelsdorf Agricultural Academy near Bonn, until he was appointed as assistant government veterinary bacteriologist of the Transvaal under Arnold Theiler* in November 1906.
Theiler arranged for him to be trained in the preparation of rabies vaccine by Jules Bordet* at the Pasteur Institute in Brussels during the first three months of 1907. He started the production of this vaccine at the Daspoort laboratories with the Pasteur vaccine strain later that year. When new laboratories were completed at Onderstepoort he was put in charge of the physical laboratory, doing physical-chemical research related to diseases such as horsesickness, piroplasmosis and lamsiekte. This work was published in "Physical-chemical investigations into South African diseases" (Annual Report of the Government Veterinary Bacteriologist of the Transvaal, 1907/8, pp. 127-211), "Investigations into the disease Lamziekte of cattle" (Ibid, 1908/9, pp. 100-115), and in two papers in the Zeitschrift fuer Infektionskrankheiten in 1909.
In January 1908 Frei became a foundation member of the Transvaal Biological Society and during that year read two papers before its members, dealing with the viscosity of blood and surface tension of serum. These formed the basis of two papers by him in the Transvaal Medical Journal the same year. Also in 1908 he read a paper titled "physical chemistry and veterinary science" at the annual congress of the South African Association for the Advancement of Science (Report, pp. 175-193) in which he reviewed the physical laws pertaining to liquids, solutions and colloids. This was followed by his "Remarks on some experiments with snake venom", published in the first volume of the Transactions of the Royal Society of South Africa (1908-1910, pp. 335-343).
Despite these publications Frei's work was not highly regarded and he returned to Switzerland after his contract expired in October 1910. The next year he was appointed Director of the Veterinary Pathology Institute in Zurich, a post he retained until his retirement in 1952. In addition to numerous journal articles he published books on the preventive treatment of contagious diseases in animals (1921), the pathology and therapy of sterility in female domestic animals (1927), and on contagion and contagious diseases in animals (1950).