I continue my brief descriptions of the major discoveries that marked the evolution of medical sciences during the first half of the 20th century. In 1933, English physician Wilson Smith was the first to demonstrate that influenza is caused by a virus. Research he conducted on ferrets proved that the flu is an infectious disease common to both animals and humans and that its causal agent was not retained by porcelain filters due to its small size. A few years later, with the advent of electron microscopy in the early 1940s, it was verified that the influenza virus measures, on average, 100 nanometres. In 1937, American physician Joseph Stokes, Jr. prepared the first flu vaccine and emphasised the importance of seasonal immunisation. That same year, South African scientist Max Theiler discovered the yellow fever vaccine, for which he received the Nobel Prize. In 1939, Swiss chemist Paul Müller announced the use of DDT as an insecticide, earning him a Nobel Prize. In 1943, Ukrainian biochemist Selman Waksman announced the discovery of streptomycin, the first truly effective antibiotic for treating tuberculosis. From 1947, the widespread use of chloroquine for the prevention and treatment of malaria began, and in 1949, American scientist John Enders successfully cultivated the polio virus in the laboratory, paving the way for the future vaccine.