Centers for Disease Control and Prevention:

 CDC Mission: To promote health and quality of life by preventing and controlling disease, injury and disability.  CDC Core Values include Accountability, Respect and Integrity.  

This is the "Gold Standard" granddaddy authority of health sites. This site provides a plethora of accurate and up-to-date authority regarding epidemiology, statistics on infections, epidemics, vaccine guidelines, accidents and other current world health issues.  Excellent site for both medical professionals and lay persons.  (Them Yankees are good!)