Data from CDC’s National Notifiable Diseases Surveillance System show that 96,075 diseases caused by flea, tick, and mosquito bites were reported in 2016, compared to just 27,388 diseases in 2004.

There are several factors that have led to the rise in diseases carried by insects over this time period, including growing populations of the creatures that transmit them, known as “vectors,” and more foreign exposure by travelers who then unknowingly bring diseases back into the United States, according to the CDC.

The significant increase in the insect-borne diseases was due in part to the outbreak of Zika, the mosquito-borne disease that can cause severe birth defects. Zika accounted for 41,680 of the cases reported in 2016.

Climate change also contributed to a steep rise in the number of reported diseases, with higher temperatures and shorter winters boosting populations of ticks, mosquitoes, and other vector species.

“It enables these ticks to expand to new areas. Where there are ticks, there comes diseases,” Lyle Petersen, director of the CDC’s Division of Vector-Borne Diseases told Reuters.

More than 80% of vector-control organizations across the United States lack the resources they need to stop the spread of the illnesses these species carry. Federal programs are increasing funding for those organizations, Peterson said.

Cases of Lyme disease, a tick-borne illness, are also rising. There were 36,429 Lyme disease cases reported in 2016 — nearly twice the number reported in 2004, according to the CDC.

Researchers note that since not all cases are reported, these figures do not capture the total number of Americans infected by ticks, mosquitoes and fleas.

“It’s very important that the public is very aware that these are more than summertime nuisances — you can get very severe diseases from ticks and mosquitoes,” Petersen, who had West Nile virus from a mosquito bite in 2003, told CNN.