Warm weather brings bugs—and the pathogens they carry. West Nile virus (blue circles), transmitted by mosquitoes, has spread from only three U.S. states in 2000 to 48 states in 2012, and human cases have climbed from 21 to 5,674. Lyme disease was concentrated in the Northeast in 2000, but cases of the bacterial infection have also picked up across the country (orange). The total U.S. number has fallen from a peak of 29,959 in 2009, however, in part because people have gotten into the habit of checking themselves and their pets for ticks.

Those illnesses can cause fever and other serious symptoms. But another, more deadly mosquito-borne disease, dengue, has recently begun to rise in the U.S. (green). In 2013 the Centers for Disease Control and Prevention recorded three cases of the virus in Texas and 20 cases in Florida. Puerto Rico, which is not listed, is a hotspot: 8,148 people there tested positive last year.

For more on dengue fever in the U.S., see ScientificAmerican.com/may2014/graphic-science