According to the U.S. National Centers for Disease Control and Prevention (CDC), there's a limited risk of dengue fever and other insect-carried or parasite-caused illnesses in some rural parts of Mexico, though Baja California Sur is not one of the major areas of concern.
If you travel a lot internationally—particularly to developing nations—refer to the CDC's Health Information for International Travel (aka Traveler's Health Yellow Book). Info from it is posted on the CDC Web site (www.cdc.gov/travel/yb), or you can buy a copy from your local bookstore for $24.95.
National Centers for Disease Control & Prevention (CDC. 877/394-8747 international travelers' health line. www.cdc.gov/travel). World Health Organization (WHO. www.who.int).