My partner A. and I came up with several possibilities in a few seconds: Louis Pasteur (helped develop the germ theory of disease), Joseph Lister (applied germ theory and advocated antiseptic techniques), Anton van Leeuwenhoek (used microscope to look at animalcules), Edward Jenner (invented smallpox vaccine), etc. etc. If I had to pick one, I'd go with Lister, since "germs=disease" is pretty fundamental to everything about medicine today, and "Hey, maybe wash your hands before you stick them inside somebody's body" is just darn good advice.
If we include basic science advances that affect medicine, then guys like Watson & Crick (discovered the double-helix structure of DNA) are eligible, and ancient guys like Galen (second-century anatomist), Vesalius (sixteenth-century anatomist; actually dissected humans and thus got better results), and Avicenna (fascinating Persian physician from the turn of the millennium who laid out some ground rules for pharmacology and clinical trials in freakin' 1025) deserve at least honorable mention.
Also learned in this conversation: as a child, A. owned a book called Great Microbiologists. Truly, we are a match made in Heaven, or like a test tube or something.
Now, here's a harder question: who is the most important woman in medical history?
We know this is a harder question for many reasons. Although an even half of medical degrees now go to women, this is a very recent development. Back when Avicenna was writing The Canon of Medicine and Galen was carving up monkeys, opportunities for women in medicine were pretty much zippo (although "medicine" in those days was less a scientific profession and more a stylized way of killing sick people with sharp things and leeches). The first openly female physician in America, Elizabeth Blackwell, didn't graduate from medical school until 1849. Throughout the twentieth century (and even today), women who do go into medicine and biomedical research have been hampered by a number of factors. Most notably, although 60% of undergraduate degrees in biology go to women, the percentage drops to 37% of biology PhDs. Steadily lower percentages of women become tenured biology professors, department heads, Nobel Prize winners, etc.
But progress is happening, slowly but surely. Ten women have won the Nobel Prize in Medicine & Physiology since the award's inception, including two in 2009, and all of their contributions to medical science are incredibly important. Due to the relatively small sample size, it's hard to isolate an individual candidate for most important woman, but there are many worthy candidates. Again, we can open it up to basic science: Rosalind Franklin (X-ray crystallography) and Marie Curie (radiation) come up a lot in lists of important female scientists.
Also, let's not forget that many women have made useful contributions despite the barriers that kept them out of the medical profession. Nursing is a devalued pink collar occupation even today, but ladies like Florence Nightingale (advocated for sanitation) and Clara Barton (founded the Red Cross) certainly deserve to be counted among important figures in medical history. What about activists like Margaret Sanger (birth control advocate), who saved countless lives? Not to mention thousands of midwives and other female healers throughout history, who surely didn't do any more harm than wacky old-timey physicians, and probably did more good. Women have been contributing to medicine for centuries, so of course it would be difficult to name a single individual as the most important woman in medical history--
Oh? What's that, Popular Science? You have a suggestion?
The February 2010 issue has a short article (page 81; couldn't find it online, so you'll have to take my word for it) with the headline "5 Reasons Henrietta Lacks is the most important woman in medical history." The name sounded familiar--was she an early physician? A Nobel laureate? An inventor? None of the above:
"In 1951, Henrietta Lacks, a poor woman with a middle-school education, made one of the greatest medical contributions ever. Her cells, taken from a cervical-cancer biopsy, become the first immortal human cell line..."
Yep. The most important woman in medical history was not a researcher or a doctor, but a test subject.
I could elaborate for several paragraphs on how messed up this is, but I would be belaboring the point. To wit: men are actors; women are acted upon. Men use; women are used. Men are minds; women are bodies. Not just bodies, but cells. Not just cells, but cancerous cells that were removed and used (without Henrietta Lacks's consent) for medical research.
Of course, we should still acknowledge Lacks's unwitting but amazing contribution. (And it's pretty amazing: Dinosaur Comics, as usual, says it better than I can.) The Popular Science article mentions contributor Rebecca Skloot's new book, The Immortal Life of Henrietta Lacks, which looks like it takes a far more measured and interesting perspective--it's not out yet, but the Amazon reviews say it discusses the "hidden costs of scientific progress" and "how easily science can do wrong, especially to the poor." These are important and valuable questions. Too bad "5 reasons the case of Henrietta Lacks exemplifies the long-standing scientific tradition of exploiting poor people of color" doesn't make quite as catchy a headline.