Major Frederick F. Russell is one of the most unknown significant figures in medicine. He was the curator of the Army Medical Museum, but also a significant bacteriologist.
In the early 1900s, he had a room in the museum converted into his personal laboratory, and after hearing about the questionable results of a typhoid vaccine used during the Boer War, he decided to develop his own, starting from scratch.
Over the course of about 5 years, his research using rabbits and bacterial cultures resulted in a promising new vaccine against typhoid fever. Animal tests went very well. In an interesting optimism about the vaccine, the workers at the Army Medical Museum (almost all of them; there were exhibit workers down to janitors) volunteered to have it tested on themselves. Luckily, it was a success, without any of the problems that the British vaccine had.
In 1910 (or 1908, depending on the source), the vaccine was available for anyone joining the military on a voluntary basis. In 1911 (or 1913, again depending on source), it was made compulsory.
In the US Civil War, over 80,000 Union soldiers died from typhoid fever or dysentery (which had very similar symptoms and were hard to differentiate before bacteriology; more soldiers are believed to have succumbed to typhoid fever than dysentery). In 1891, typhoid fever deaths were all the way up to 174 per 100,000 people. That resulted in over 130,000 civilian deaths in one year.
Within two years of the introduction of the vaccine (the second year implementing drastic sanitation regulation changes), both branches of the military were free of typhoid fever. United States deaths from typhoid fever in WWI were 80% lower than in non-vaccinated countries (France, Germany, etc…most British soldiers were vaccinated).
Major F.F. Russell eventually went on to do significant work on the Yellow Fever problem, and to win the Public Welfare Medal from the National Academy of Sciences, in 1935.