The study did not delve into the reasons for the increase, but Dr. Petersen said it was probably caused by many factors, including two related to weather: ticks thriving in regions previously too cold for them, and hot spells triggering outbreaks of mosquito-borne diseases.
Other factors, he said, include expanded human travel, suburban reforestation and a dearth of new vaccines to stop outbreaks.
More jet travel from the tropics means that previously obscure viruses like dengue and Zika are moving long distances rapidly in human blood. (By contrast, malaria and yellow fever are thought to have reached the Americas on slave ships three centuries ago.)
A good example, Dr. Petersen said, was chikungunya, which causes joint pain so severe that it is called “bending-up disease.”
In late 2013, a Southeast Asian strain arrived on the Dutch Caribbean island of St. Maarten, its first appearance in this hemisphere. Within one year, local transmission had occurred everywhere in the Americas except Canada, Chile, Peru and Bolivia.
Tickborne diseases, the report found, are rising steadily in the Northeast, the Upper Midwest and California. Ticks spread Lyme disease, anaplasmosis, babesiosis, Rocky Mountain spotted fever, rabbit fever, Powassan virus and other ills, some of them only recently discovered.
Ticks need deer or rodents as their main blood hosts, and those have increased as forests in suburbs have gotten thicker, deer hunting has waned, and rodent predators like foxes have disappeared.