Bugs In The News:  A interesting site developed by a professor at Kansas.
http://people.ku.edu/~jbrown/bugs.html

The Merck Manual:  A good general look at definitions and treatments for a variety of diseases.
http://www.merck.com/mmpe/index.html

Healh Finder:  A United States supported site to help you in medical information. 
http://www.healthfinder.gov/

Health Links:  A site for finding information and linking to further resources.
http://www.healthlinks.com/

Mayo Clinic:  Web site for the Mayo Clinic, with many helpful resources.
http://www.mayoclinic.com

Medline Plus:  A site sponsored by the National Library of Medicine.  A good place to search a specific topic.
http://www.medlineplus.gov