Farewell, carefree days of summer.

The number of people getting diseases transmitted by mosquito, tick and flea bites has more than tripled in the United States in recent years, federal health officials reported on Tuesday. Since 2004, at least nine such diseases have been discovered or newly introduced here.

The Centers for Disease Control and Prevention did not suggest that Americans drop plans for softball games or hammock snoozes. But officials emphasized that it’s increasingly important for everyone — especially children — to be protected from outdoor pests with bug repellent.

New tickborne diseases like Heartland virus are showing up in the continental United States, even as cases of Lyme disease and other established infections are growing. On island territories like Puerto Rico, the threat is mosquitoes carrying viruses like dengue and Zika.

Warmer weather is an important cause of the surge, according to the lead author of a study published in the C.D.C.’s Morbidity and Mortality Weekly Report.