Skip navigation
Harvard Health Publishing, Harvard Medical School

Terms to Know

Being familiar with the key terms on this page will help you to better understand Lyme disease.

antibodies: proteins created by the immune system to help fight off infection. Lyme disease tests look for antibodies against Borrelia, the bacteria that causes Lyme disease.

acaricide: a type of pesticide that kills ticks.

black-legged tick: the type of tick that transmits the Lyme disease bacteria. The scientific name is Ixodes scapularis (Eastern black-legged tick) and Ixodes pacificus (Western black-legged tick), and it is sometimes referred to as a deer tick.

Borrelia: the bacteria that causes Lyme disease. There are several species of Borrelia. Borrelia burgdorferi is the bacterial species most associated with Lyme disease in the United States.

co-infections: other tick-borne diseases that you can get along with, or instead of, Lyme disease.

early localized (Stage 1) Lyme disease: occurs soon after infection when Lyme disease bacteria are still close to the site of the tick bite.

early disseminated (Stage 2) Lyme disease: usually occurs weeks to months after a tick bite. Lyme disease bacteria have moved beyond the site of the tick bite to other parts of the body such as the heart, brain, or spinal cord.

enzyme-linked immunosorbent assay (ELISA) test: the first of two tests required to determine if you have Lyme disease. This blood test looks for antibodies against the Lyme disease bacteria.

endemic: regularly found in a particular area.

engorged: round and full of blood. Ticks become engorged when they feed on blood from an animal host.

erythema migrans (EM) rash: a type of rash that only occurs in people with Lyme disease. It often, but not always, has a target or “bullseye” shape. An important characteristic of EM rashes is that they expand over time. EM rash is not present in all cases of Lyme disease.

host: an animal that carries the Lyme disease bacteria and that the tick feeds on. The bacteria may not cause illness in all hosts.

Ixodes scapularis: the scientific name for black-legged ticks found in the Eastern U.S. Also known as deer ticks. One of two tick species in the U.S. associated with Lyme disease.

Ixodes pacificus: the scientific name for Western black-legged ticks, found in the Western U.S. Also known as Western black-legged ticks. One of two tick species in the U.S. associated with Lyme disease.

late disseminated (Stage 3) Lyme disease: the infection can spread to the joints or contribute to altered brain function. Late disseminated Lyme disease usually occurs months to years after a tick bite.

pathogen: disease-causing microbe.

permethrin: a pesticide that can be used on clothing and gear to kill ticks and mosquitos on contact.

prophylactic antibiotics: also called preventive antibiotics, these are antibiotics given to prevent illness.

questing: a host-seeking strategy used by ticks such as the black-legged tick. The tick climbs up vegetation and waits with its upper legs outstretched for an animal or person to pass by.

repellent: a substance that makes you less attractive to ticks and insects. Most tick repellents contain DEET, picaridin, or natural repellents.

spirochete: a spiral or corkscrew-shaped bacteria. The Lyme disease bacteria, Borrelia, is a spirochete.

tick check: a full, thorough scan, from scalp to toes, for ticks. Ticks gravitate towards warm, moist areas, such as the belly button, armpits, groin, behind the knees, behind the ears, and the back of the neck.

transmission time: the amount of time a tick needs to be attached to transmit the pathogens that cause Lyme disease and other diseases.

two-tiered test: two-step blood test for Lyme disease. If the first test (an ELISA test) comes back positive or unclear, the second blood test (either a second ELISA or a Western blot test) is done.

vector: an organism that transmits a pathogen from an infected animal to a human or other animal. Ticks are vectors for Lyme disease bacteria.

Western blot test: an option for the second blood test in a two-tiered Lyme disease test. The Western blot test looks at how your antibodies react to specific parts of the Lyme disease bacteria.