If 2023 seems to you like an especially bad year for ticks, it’s not your imagination. “From 2020 to this year, I would say it’s a 100% increase in the number of ticks humans have encountered,” Saravanan Thangamani, professor in the department of microbiology and immunology at SUNY Upstate Medical University, told me.

That, as you might imagine, is bad news, because the U.S. Centers for Disease Control and Prevention lists no fewer than 16 diseases ticks carry that can be dangerous—or even deadly—to humans and animals. I spoke with a number of experts to learn what's behind the tick boom. Based on those conversations, here are the three most important forces at play:

• Climate change: Brief, mild winters and long, hot springs and summers are incubators for ticks, especially in the Northeast and the Midwest, which once featured punishingly cold winters, but increasingly do not. Rising temperatures also affect tick migration: the Gulf Coast tick, for example, once found only along the Gulf of Mexico, is now found as far north as New York and New Jersey.
• Human habitats: As housing developments sprawl into once-green spaces, people are coming into greater contact with tick-carrying animals like white-tailed deer and white-footed mice.
• Stowaway ticks: Ticks don’t recognize borders, and will cross readily from state to state and country to country. Lax inspection of cattle entering the U.S. from other countries allows ticks to slip through. Dogs arriving from other countries must be shown to have had their rabies shots, but they are not inspected at all for ticks.

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