Roughly 15 to 25 percent of deer ticks carry the bacterium that causes Lyme disease, depending on life stage, location, and year. In the highest-risk areas of the northeastern and upper midwestern United States, infection rates in ticks can push past 20 percent, while ticks in lower-risk regions may carry the bacterium at rates in the single digits.
Infection Rates by Region
Not all deer ticks are equally dangerous. A multi-state study of nymphal deer ticks collected from residential properties in Connecticut, Maryland, and New York found overall infection rates of 18.5 percent in one year and 15.3 percent the next. But those averages masked real variation: infection rates at individual sites ranged from about 9 percent in parts of Connecticut to over 23 percent in New York.
Adult deer ticks tend to carry the Lyme bacterium at somewhat higher rates than nymphs, often in the 20 to 30 percent range in endemic areas. That said, nymphs are responsible for most human infections. They’re tiny (about the size of a poppy seed), active during the summer months when people spend the most time outdoors, and far easier to miss on the skin.
Deer Ticks Carry More Than Just Lyme
The Lyme bacterium isn’t the only pathogen living inside deer ticks. A study of nearly 400 adult deer ticks from Indiana, Maine, Pennsylvania, and Wisconsin found that 89 percent were infected with at least one microorganism. Among those infected ticks, 45 percent carried two or more pathogens simultaneously. The most common co-infections included the agents that cause anaplasmosis and babesiosis, two other tick-borne diseases that can produce fever, fatigue, and in the case of babesiosis, destruction of red blood cells.
In the nymphal tick study from Connecticut, Maryland, and New York, the pathogen causing anaplasmosis showed up at rates of 1.5 to 4.8 percent. The parasite behind babesiosis ranged from about 6 percent to over 15 percent depending on the site and year. These co-infections matter because a single tick bite can potentially deliver more than one disease at a time, which can complicate symptoms and diagnosis.
Why Attachment Time Matters More Than the Percentage
Even if a tick is carrying the Lyme bacterium, transmission isn’t instant. The bacteria live in the tick’s gut, not its saliva. When a tick begins feeding, the bacteria multiply dramatically (increasing more than 300-fold over about 72 hours) and migrate from the gut through the tick’s body to its salivary glands. Only then can they pass into your bloodstream.
This migration takes time, which is why the CDC states that an infected tick generally must be attached for more than 24 hours before it can transmit the Lyme bacterium. A tick you find and remove the same day is unlikely to have transmitted the infection, even if it was carrying the pathogen. This is the single most practical takeaway: daily tick checks after spending time in wooded or grassy areas are one of the most effective ways to prevent Lyme disease.
What a Tick Bite Looks Like Afterward
If you find an engorged tick on your body, one visibly swollen with blood, it has been feeding for a significant amount of time and the risk of transmission goes up. A flat, unfed tick likely attached recently and is much less likely to have passed along any pathogens.
After removing a tick, a small red bump at the bite site is normal and doesn’t mean you have Lyme disease. The signature bull’s-eye rash, a red ring that expands outward from the bite over days or weeks, appears in roughly 70 to 80 percent of confirmed Lyme cases. Other early symptoms include fever, fatigue, headache, and muscle aches, typically starting within 3 to 30 days after the bite.
Preventive Treatment After a Bite
If you were bitten in an area where Lyme disease is common, removed the tick within the last 72 hours, and the tick was engorged, a single preventive dose of an antibiotic can significantly reduce your risk of developing Lyme disease. The treatment works best within that 72-hour window after tick removal because the Lyme bacterium has an incubation period of at least three days. Your doctor will consider whether the tick was a deer tick (as opposed to the many tick species that don’t carry Lyme) and whether you’re in a geographic area where the bacterium is circulating in the tick population.
Putting the Numbers in Context
Over 89,000 cases of Lyme disease were reported to the CDC in 2023, but that figure dramatically undercounts the real burden. The CDC estimates that approximately 476,000 people are diagnosed and treated for Lyme disease each year in the United States. The gap between reported cases and actual cases reflects incomplete reporting, cases treated based on clinical judgment without lab confirmation, and the sheer volume of tick encounters in endemic regions.
So while only about one in five or six deer ticks carries the Lyme bacterium in high-risk areas, the combination of abundant ticks, frequent human exposure, and a pathogen that can cause lasting joint, heart, and neurological problems if untreated makes Lyme disease the most common vector-borne illness in the country. The percentage of infected ticks in your specific area is what matters most, and local health departments often publish surveillance data that can give you a clearer picture of risk where you live.