Puerto Rico is in the midst of a spike in cases of Dengue fever. The latest report by the CDC indicates a higher-than-normal number of cases thus far in 2010. While this is no reason to panic (Dengue is a prevantable disease), travelers to the island may want to be aware of this latest development. For starters, you might want to familiarize yourself with the symptoms of the sickness.
Puerto Rico has historically experienced Dengue activity since the 1960s, so this is nothing new on the island, and really, in all of the Caribbean. As such, I wouldn't (and never have) cancelled or changed my plans due to an increase in cases, but for those who want to play it extra-safe, I'd recommend checking the website of the Dengue branch of the Center for Disease Control (CDC) before you travel. The branch is located in San Juan and has up-to-date information for travelers.