Travelers heading to parts of Uruguay, Paraguay (Feb 2008 yellow fever alert - read more) and Argentina should have a look at the CDC site (Center for Disease Control for current information about an outbreak of dengue fever. This outbreak underscores the importance of consulting a good travel clinic before setting out on any trip. Local conditions can change fairly rapidly, so it's good to be prepared for the usual suspects.
Most countries will issue travel advisories for their citizens (see post January 20, 2007 ) so check the links there for current information.
As I understand it, the virus that gives you dengue fever is transmitted by mosquitoes, and found in subtropical areas around the world. Dengue can be deadly, and unlike malaria, there's no vaccine or prophylactic treatment, so travel safe, travel smart! Check with a travel clinic before heading south.