So what we’re worried about this summer goes back to 2015, which was one of those big acorn years; “2016 was the biggest mouse year we’ve seen in our entire 25 years of monitoring them,” Dr. Ostfeld said.

“We’re likely to see more ticks, and a lot of us have already started seeing more tick-borne infections,” said Dr. Nicholas Bennett, the head of the division of pediatric infectious diseases and immunology at Connecticut Children’s Medical Center.

“Far and away, the most common infection I see is Lyme,” Dr. Bennett said. The bacteria is present in many ticks — more than half of them in some areas — but only about 3 percent of tick bites result in infection. Once the tick attaches and starts feeding, the bacteria have to undergo maturation within the tick before transmission is possible, so it takes at least 36 hours from the time an infected tick bites until Lyme disease can be transmitted, which is why people are told not to worry about Lyme if they find and remove a tick soon after a child has been outside. (On the other hand, if you do find that tick, it reminds you that you’re in an endemic area and may have had other bites.)

The classic symptom of Lyme disease is the bull’s-eye rash called erythema migrans. But the diagnosis is not always obvious. “Half the people who have clear Lyme don’t remember getting bitten and never have a rash,” Dr. Bennett said.

If a child doesn’t get better with the usual antibiotic treatment, doctors may suspect a second infection, like babesiosis or anaplasmosis, since some ticks carry more than one disease, and since the first-line antibiotics we use in children for Lyme don’t treat the other tick-borne infections. Anaplasmosis and babesiosis are less common, Dr. Bennett said, though regional infection rates may vary. And Powassan, though it is the rarest, is scary because it can be transmitted so quickly, and because, since it is a viral infection, there is no antibiotic treatment. Detailed information on all the tick-borne diseases is available on the website of the Centers for Disease Control and Prevention.