There are a lot more than these five medical search engines out there; however, I've found these ones to be some of the most useful medical search engines that I've come across. Use these medical search engines to find answers to your medical questions, get more information about various health topics, or just to learn about something new.
OmniMedicalSearch at the time of this writing brings back search results from 30 different sources, and you can search up to 12 different medical search engines at one time. You can focus your search using the dropdown menu on the home page; in addition, OmniMedicalSearch has tabbed search options for more medical search information: News, Images, and Web2, which searches only specific domains among four major search engines.
Probably the very first place I've always gone on the Web for medical information, WebMD is a great one-stop medical information site. WebMd also has a lot of interesting interactive calculators, quizzes, and other fun stuff that helps you understand medical information a bit easier.
PubMed is a really, really extensive medical search engine/database that is a service of the National Library of Medicine. Over 15 million MEDLINE articles and journal citations are available here to search through. Depending on the article, you can view an abstract or full-text. Some articles are available for purchase as well.
Healthline is relatively new, but I expect that the quality of medical search results that I've already seen as detailed in my profile will only continue to grow and improve. Healthline offers a few unique features, including physician-filtered results and clustered "healthmaps".
This is a great medical and health information site put together by the US Department of Health and Human Services. You can search through over 1500 health-related organizations at the time of this writing, and I found the search process to be extremely user-friendly and relevant.