"Greatest experiment" in microbiology/infectious disease

I'm late to the party, but Chad over at Uncertain Principles put out a call for great experiments/observations in our fields. Like others have said, that's a tough one, so I thought I'd first run through some of the highlights and big breakthroughs in the fields of microbiology and infectious disease epidemiology that have made the field what it is today, and then end with the one I think is most important. Feel free to disagree.

Obviously, several early advances stand out that allowed for the field to really start to develop. In roughly chronological order:

Antony van Leeuwenhoek's initial description of bacteria and protozoa and use of microscopy.

Jenner's introduction of vaccination (and others who had pioneered variolation), showing that immunity to a disease could be induced.

John Snow's contribution of linking cholera in London to a well, suggesting the disease was water-borne. His use of disease mapping, stastics, and the case/control study design to examine risk factors for disease are still essential elements of epidemiological studies.

Numerous experiments by Pasteur: showing that lactic acid fermentation is due to bacteria; that microbes don't arise by spontaneous generation; vaccines against rabies and anthrax, demonstrating that organisms could be attenuated and made less virulent; spreading the idea that germs cause disease.

Obviously, Charles Darwin for laying the groundwork to tie everything together.

Joseph Lister and the practice of disinfection.

Again, several advances by Robert Koch: in addition to Koch's postulates for determining disease causation, Koch also developed solid growth media for bacteria.

Paul Ehrlich's discovery of Salvarsan for syphillis and Fleming's discovery of penicillin, leading to widespread use of antibiotics.

From here, the advances in the field shift more to molecular genetics. Obviously, a hallmark was the use of Streptococus pneumoniae by Avery, MacLeod, and McCarty to show that DNA was the material of heredity. (Interestingly, one pupil of Avery's was Rebecca Lancefield, who developed the typing system used for streptococci--group A, B, etc.--and went on to become president of the American Society for Microbiology).

Luria and Delbruck's work on the nature of mutations.

Josh Lederberg and the discovery of plasmids and the process of conjugation. This really led to the founding of the field of microbial genetics.

The discovery of restriction enzymes by Nathans, Arber, and Smith. This has been especially important in molecular epidemiology of infectious diseases--much of typing of organisms is still due to patterns produced when their DNA is cut with restriction enzymes. (Come to think of it, serotyping is another huge advance in infectious disease epi--and one that I've never seen touted in these kind of introductions. Something to investigate...) Restriction enzymes have also been used for gene cloning, forensic work, many other applications.

Roy Anderson and Robert May--mathematical biology, and modeling spread of infectious diseases.

If anyone's versed in the history of medicine, you may have noticed one I missed. Rather than point to the usual suspects of Koch, Jenner, Pasteur et al., if I had to pick out experiment that has had the most impact, it would be the work of Ignaz Semmelweis. Semmelweis was a physician in Vienna in the 1840s, with an interested in "childbed fever," a leading cause of mortality in women who'd given birth. During this time, he noticed that the mortality rate from this disease in a hospital division where medical students delivered babies was 16%, while in a division where midwives delivered them was ~2%. It was also known that childbed fever was rare when women gave birth at home. Semmelweis thought there was something the med students were doing that served to raise the rates of childbed fever in those divisions.

In 1847, Semmelweis' friend, another physician, died due to a wound acquired while performing an autopsy. Semmelweis examined the tissues of his friend, and noticed the pathology there was similar to those in women who'd died of childbed fever. According to history, this led to his "eureka" moment: medical students performed autopsies, and midwives did not. The students must be bringing some contagious agent from the autopsy room back to the delivery room.

To test this, Semmelweis instituted a procedure, requiring students to wash their hands in a chlorine solution before entering the maternity ward. Mortality dropped dramatically, and Semmelweis extended the procedure to include surgical instruments as well. However, colleagues scoffed. Semmelweis actually lost his job, and took a position in Budapest--where he again instituted his handwashing protocol, with similar incredible results. Sadly, he died in 1865 in an asylum, disgraced.

Pasteur later identified the bacterium as Streptococcus pyogenes (group A streptococcus), and presented the example of S. pyogenes as the cause of childbed fever as support for the germ theory of disease. Thus, while Semmelweis' ideas were not popular in his life, handwashing still remains the best way to prevent disease due to microbes, and disinfection (later built upon by Lister) has made surgery and other procedures orders of magnitude safer. Obviously, vaccination has saved millions of lives--but despite a century of intense study, vaccines are only available for precious few diseases. Studies by Koch and Pasteur have enabled us to determine microbial cause of a number of diseases and advanced the field immensely. But basic hygiene affects us--and protects us--every day, from an incredible diversity of potential pathogens. So, that's my pick for the "greatest experiment" in my field.