The inputs
The score is a comprehensive hazard over the ticks that bite people here: the deer tick, the American dog tick, the lone star tick, the Gulf Coast tick, and the brown dog tick. Each contributes on its own seasonal calendar, habitat, local presence, and disease severity, and a shared weather gate can suppress all of them. The deer tick dominates almost everywhere; the others add shoulders where they are established (lone star and Gulf Coast on the coast and inland edges, the brown dog tick around homes and yards). Five signals set the number.
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Current weatherOpen-Meteo
Ticks quest when it is roughly above 45°F in humid air with moist ground, and they retreat when the air is dry or the week has been rainless. We read each town's temperature, how thirsty the air is, and the last seven days of rain, plus satellite soil moisture where it is available. For the humidity piece we use vapor-pressure deficit rather than raw relative humidity, because drying out is what limits a questing tick and the two are not the same: the same 60% humidity dries a tick out far faster on a hot afternoon than on a cool morning. A deer-tick nymph, about the size of a poppy seed and the stage behind most Lyme cases, loses water quickly and cannot stay out long in thirsty air. Weather is a shared gate applied to every tick: a deep freeze or a hot dry spell pushes the whole score toward zero. It is refreshed every day, drives the 7-day outlook, and is often what makes two nearby towns differ from one day to the next.
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Each tick's seasonal activitySeasonal calendars
The ticks peak at different times, so each rides its own curve. Nymphal deer ticks peak in late spring and early summer (and cause most human Lyme cases), with a smaller adult peak in fall. American dog-tick adults are active from late spring through summer. Lone star ticks add a distinct late-summer shoulder, when their aggressive larval "seed tick" swarms are out. Folding lone star into the deer-tick curve would count it in the wrong season, so we keep the calendars separate.
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Forest, edge, and open coverESA WorldCover
From 10-meter satellite land cover (ESA WorldCover, built from the European Space Agency's Sentinel satellites) we measure each town's forest cover, its forest-edge density, and its open grassy/brushy cover. Every cell is 10 meters on a side, more than 20 billion of them across the states we cover. Different ticks favor different ground: deer ticks live in dense forest and its leaf litter and meet people at the woods-lawn edge; dog ticks prefer open, grassy areas and trail edges; lone star ticks favor brushy edges and coastal scrub. Edge carries the most weight because it concentrates the whole cycle in one narrow band: the mice and other small mammals that infect ticks with the Lyme bacterium live there, the deer that adult ticks feed on to reproduce travel along it, and it is where people walk. This is the part of the score that is truly local to your town, and it is what separates a heavily wooded town from a developed one in the same county.
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Local disease rateU.S. CDC
The score is anchored to how much tick-borne disease a place actually reports, and which disease that is depends on the region. Across the East and Midwest it is Lyme: the county baseline starts from annual Lyme-disease incidence per 100,000 people (CDC county surveillance) plus a state-level uplift for babesiosis and anaplasmosis. In the West, where Lyme is rare, we substitute the diseases that actually occur there, spotted fever (Rocky Mountain spotted fever and its relatives), Colorado tick fever, and tularemia, so the Rocky Mountain wood tick is incidence-backed the same way the deer tick is in the East. Every town in a county or state shares this figure; weather and habitat set towns apart within it.
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Which ticks live here, and severityU.S. CDC
CDC county surveillance records whether each tick is established or reported locally. That presence gates the dog and lone star contributions: where a tick is not established, it adds little or nothing (this is what keeps lone star risk out of inland towns where it does not live). Because dog-tick and lone-star diseases (Rocky Mountain spotted fever, ehrlichiosis, alpha-gal syndrome, and others) are not cleanly reported per county, they enter as a documented severity weight, not a case rate. A county with no record is a surveillance gap, not proof a tick is absent.
How they combine
Weather comes first, as a shared gate: a deep freeze or a hot dry spell pushes the score toward zero no matter which ticks live nearby. When conditions do let ticks out, each species adds its own contribution from its seasonal activity, its habitat in the town, how established it is locally, and its disease severity. The deer tick, incidence-backed, is the dominant term almost everywhere; the American dog tick adds a modest warm-season shoulder, the lone star and Gulf Coast ticks add late-summer shoulders in the coastal and inland- edge places where they are established, and the brown dog tick adds a small shoulder in developed, residential areas where it lives around homes and dogs. Those contributions are summed, scaled to 0 to 100, and sorted into the three bands. Near the very top of the scale we ease the final stretch so the most extreme towns do not all pile onto a flat 100, which would hide the real differences between them; the rest of the range is untouched.
Every town page shows the split, so you can see how much of the day's number is each tick, and that the rarer ticks are modeled on presence and activity rather than case counts.
The West is a different tick country
The score is built the same way everywhere; two of its inputs, the vector and the disease rate, just take on their local values west of the Rockies. On the Pacific slope of California, Oregon, and Washington the Lyme vector is the western blacklegged tick (Ixodes pacificus), a different species from the eastern deer tick that lives in oak woodland and coastal brush and peaks in spring rather than mid-summer, so we label and time it as itself. Across the arid interior and the mountains the tick that actually bites people is the Rocky Mountain wood tick (Dermacentor andersoni), which quests in spring from shrub-steppe, rocky slopes, and grassland-forest edge, roughly 4,000 to 10,500 feet, so we score it on that elevation band and its spring calendar. In California's chaparral we add the Pacific Coast tick (Dermacentor occidentalis), the state's most commonly encountered human-biting tick.
The disease-rate input changes with them. Lyme is rare in the West, so it would be the wrong number to lean on; instead we use the diseases the region actually reports, spotted fever (Rocky Mountain spotted fever and its relatives), Colorado tick fever, and tularemia, as the incidence backing for the wood tick, exactly the role Lyme plays for the deer tick in the East. That is why a place like Arizona or Colorado, with a real spotted-fever and Colorado-tick-fever burden, reads higher than the wetter Pacific Northwest. We also stop rewarding humid air in the arid West, where the moisture relationship differs, and we describe the desert Southwest's peridomestic brown dog tick (a spotted-fever vector around homes and dogs) in words. Soft ticks behind tick-borne relapsing fever live in rodent-infested mountain cabins rather than questing in the landscape, so they are noted but not scored.
Alaska and Hawaii are their own cases. Hawaii has no established Lyme tick; its one tick of note is the peridomestic brown dog tick, so its scores are low and about dogs rather than the outdoors. Alaska now has the American dog tick and brown dog tick establishing in populated areas alongside native ticks that rarely bite people, but pathogen testing has so far found nothing dangerous in Alaska's ticks, so its risk stays very low and mostly travel-associated. In both states the blacklegged (Lyme) tick is not established, so that term is switched off rather than letting forest cover imply a Lyme risk that is not there.
Limits and caveats
This is a defensible heuristic, not a validated epidemiological model. It estimates how favorable conditions are for tick activity in a place on a given day; it cannot tell you whether a specific tick carries disease. County disease rates are per-capita and annual, so they describe a county's general burden, not your individual risk, and land cover reflects a town's broad make-up, not your yard.
What counts as a town. We score every town and city we can tie to a county, 32,386 across 51 states. In the Northeast that means 2023 Census county subdivisions: the towns and cities of New England (plus Maine's plantations and tribal reservations), New York's towns, cities, and reservations, and every New Jersey and Pennsylvania municipality. Across the South, Midwest, and West, where county subdivisions are not active governments, we use Census places instead: incorporated cities and towns plus the larger census-designated places, which is how much of the unincorporated West is captured. Unorganized territory and uninhabited fragments are excluded, and a place that straddles a county line is scored once per county piece, because the disease signal is county-based. We say "towns" throughout the site as shorthand for all of these.
Not medical advice. If you find an attached tick or develop symptoms after a bite, contact a healthcare professional and see CDC tick-bite guidance.