As announced, Google Health is live. You can try it only here for the moment.
Kosmix has a similar structure with Google Health, but has more subcategories (targets specific patients: kids, women), it lets you personalize the search and has much less results than Google (for [bird flu], Kosmix has 17,506 results).
WebMD has less categories than Kosmix and Google Health, their ranking algorithm isn't that good (an overview for bird flu is ranked #8, while the first results are about the shortage of flu shots). WebMD has a very good video archive and many interesting articles, but it has too few results (for [bird flu], it has 956 results).
Google integrated Google Health in their web search, but the categories aren't very well delimited and are easy to ignore. Also if you select a category, the Cache version doesn't work for any result (this seems to be a bug).
All in all, I think Kosmix and Google Health do a better job than WebMD, but Google Health wins because it has much more content.