What Is Lyme Disease?
Definition and Overview
Lyme disease is a tick-borne illness caused by the spiral-shaped bacterium Borrelia burgdorferi — and, less commonly in North America, Borrelia mayonii. These bacteria are transmitted to humans through the bite of infected black-legged ticks, also known as deer ticks (Ixodes scapularis in the East and Midwest, and Ixodes pacificus on the West Coast).
The disease was first formally identified in 1975 following an unusual cluster of arthritis cases in Lyme, Connecticut. Since then, research has revealed its true complexity — from the bacteria's ability to evade immune detection to its multisystem effects that can mimic dozens of other conditions.
What Causes Lyme Disease?
Borrelia burgdorferi is a spirochete — a corkscrew-shaped bacterium that has evolved remarkable abilities to survive in diverse hosts and evade immune responses. The bacteria live primarily in small mammals, especially white-footed mice, which serve as the main reservoir. Ticks pick up the bacteria during feeding, then transmit it to humans and other animals in subsequent feedings.
Not All Ticks Carry Lyme
Only black-legged ticks transmit Lyme disease. The American dog tick and lone star tick — commonly mistaken for deer ticks — do NOT transmit Lyme, though they spread other illnesses. See our Tick Identification guide.
Who Is at Risk?
Anyone who spends time in wooded, brushy, or grassy areas can be exposed. Highest-risk groups include:
- People who live in or visit forested areas of the Northeast, Mid-Atlantic states, and upper Midwest
- Children ages 5–14, who play outdoors in tick habitat during peak season
- Adults ages 40–70, particularly those who hike, garden, or work outdoors
- Outdoor workers in landscaping, forestry, wildlife management, and construction
- Pet owners whose animals may carry ticks into the home
Common Myths — Corrected
- Myth: You always get a bull's-eye rash. Fact: The characteristic rash appears in only 70–80% of cases and may not have a clear bull's-eye pattern.
- Myth: Lyme is easy to test for. Fact: Standard tests have significant limitations, especially in the first weeks of infection.
- Myth: Lyme only occurs in summer. Fact: Ticks are active any time temperatures exceed 35°F — which can mean winter activity in mild regions.
- Myth: Once treated, you're cured. Fact: 10–20% of patients continue to experience symptoms after standard treatment.