U.S. Threat Level
High. Of the hundreds of cases documented in dozens of states, most were picked up internationally. Experts think it's just a matter of time before there's a larger outbreak within the United States.

How It Spreads 
Mosquitoes

What It Does 
Called "breakbone" fever because it can cause severe joint pain, dengue comes with high fever and headaches, eye pain, rashes, and bleeding.