Lyme disease season peaks in late spring and early summer, with June typically the single highest-risk month in the United States. About 54% of all reported cases in 2021 had onset dates in May, June, or July, according to CDC surveillance data. But the full window of risk stretches well beyond those three months, depending on where you live and which life stage of tick is active.
Why May Through July Is the Danger Zone
The bacteria that cause Lyme disease are primarily spread by nymphal ticks, the juvenile stage of the black-legged tick (commonly called the deer tick). Nymphs are roughly the size of a poppy seed, making them extremely difficult to spot on your skin. In the Northeast, these nymphs are most active from May through July, and that timing lines up almost exactly with the annual spike in Lyme disease cases.
CDC data from 2021 shows the monthly pattern clearly: cases climb sharply in May (4,509 reported), peak in June (4,970), then drop by more than half in July (2,013). By August, cases fall further to around 1,048. The reason June dominates isn’t just tick activity. It’s also when people start spending more time outdoors in shorts and sandals, increasing skin exposure right when nymphs are at their hungriest.
Research on tick collection rates reinforces this timing. In a Massachusetts study, the number of infected ticks collected per person-hour peaked in May, stayed high in June, then dropped notably through July, August, and September. So while summer feels like “tick season,” the true transmission risk is concentrated in that late spring window before most people are thinking about it.
The Second Season Most People Miss
Adult black-legged ticks have their own activity period that runs from October through May, as long as daytime temperatures stay above freezing. These adults are larger and easier to spot than nymphs, so they’re less likely to feed undetected long enough to transmit Lyme bacteria (transmission typically requires the tick to be attached for 36 to 48 hours). Still, adult ticks absolutely can and do transmit Lyme disease, particularly in fall and early spring when people assume tick season is over.
This means there’s effectively no month of the year when Lyme disease risk is truly zero. The 2021 CDC data recorded cases in every single month, including 262 in January and 320 in December. These numbers are small compared to the June peak, but they’re not negligible.
How Temperature Drives Tick Activity
Ticks don’t operate on a calendar. They respond to temperature. Black-legged ticks become active whenever the ground is thawed and the air temperature rises above freezing. A warm spell in February can bring them out just as easily as a day in May. Once temperatures drop and the ground freezes solid, ticks lose the ability to move their muscles, but they aren’t killed. Warm them up even slightly and they’re immediately mobile again.
This is why mild winters and early springs can shift the start of Lyme season earlier. Snow cover actually helps suppress tick activity by insulating the frozen ground and keeping ticks dormant beneath it. A snowless winter with frequent above-freezing days creates more opportunities for tick encounters than a consistently cold, snowy one.
Regional Differences Across the U.S.
The classic May-through-July peak applies primarily to the Northeast and Mid-Atlantic states, where the vast majority of U.S. Lyme cases occur. The Upper Midwest (Minnesota, Wisconsin) also sees high case counts, but tick behavior there differs in a notable way. In the Northeast, nymphs and larvae feed during distinct, separate windows. In the Upper Midwest, nymphal and larval feeding seasons overlap, which can subtly shift the timing and intensity of local risk.
The West Coast has its own pattern entirely. The western black-legged tick, the species responsible for Lyme transmission in California, Oregon, and Washington, follows a different schedule. Adults become abundant in early December and stay active through winter into early summer, when dropping humidity shuts them down. Nymphs on the West Coast peak in April, May, and June, roughly similar to the eastern timeline but shifted slightly earlier. If you live in Northern California or the Pacific Northwest, your highest-risk months for Lyme may start in late winter rather than late spring.
Practical Timing for Prevention
If you’re in the Northeast or Midwest, your highest-alert period should begin in April and run through July. May and June deserve the most caution, particularly if you’re hiking, gardening, or spending time in wooded or grassy areas. Tick checks after outdoor activity are most critical during these months, but they shouldn’t stop when summer winds down.
For fall hikers and hunters, October through November marks the start of adult tick season. The risk per encounter is lower because adult ticks are easier to find on your body, but you should still check thoroughly after time in tick habitat. Pet owners need year-round awareness as well, since dogs can pick up ticks on any above-freezing day and bring them into the home.
On the West Coast, shift your vigilance earlier. December through June covers the full window when either adult or nymphal ticks are actively seeking hosts. The nymphal peak in April through June is when transmission risk is greatest, since those tiny nymphs are hardest to detect.
The simplest rule: if the temperature is above freezing and you’re in an area where Lyme disease is endemic, ticks can be active. The poppy-seed-sized nymphs of late spring are the ones most likely to make you sick, but the risk never fully disappears until the ground is frozen solid and covered in snow.