[1][2] in 1900, Watson entered the Medical Service of the Federated Malay States, and the following year was posted to Klang as district surgeon.

It was here that Watson first put into practice measures to combat malaria following the discovery by Ronald Ross in 1897 that the disease was transmitted by mosquitoes.

In Klang, and in many other areas of Malaya, the population had been devastated by malaria, and he began a vigorous program of mosquito control, introducing radical new methods to prevent larval reproduction in different environments.

Described as the "man who conquered malaria", Ross acclaimed his work in Malaya as "the greatest sanitary achievement ever accomplished in the British Empire.

"[4][6][5] In 1927, Watson left Malaya having spent seven years in government service and 21 years as a private consultant working in malarial research and prevention, and at the request of Ronald Ross joined, as director of the malaria department, the newly created Ross Institute of Tropical Health in Putney Heath, London.