Ticks are the leading carriers (vectors) of diseases to humans in the United States, second only to mosquitoes worldwide. It is not the tick bite but the toxins, secretions, or organisms in the tick's saliva transmitted through the bite that causes disease.
The following is a list of tick-borne diseases, the usual tick vector(s), and the pathogen(s) the tick transmits:
- Lyme disease (borreliosis) -- Ixodes species including deer ticks (hard ticks) -- vectors for Borrelia species of bacteria (a spirochete or spiral-shaped bacterium)
- Babesiosis -- Ixodes species (hard ticks) -- vectors for Babesia, a protozoan
- Ehrlichiosis -- Amblyomma americanum or lone star ticks (hard ticks) -- vectors for Ehrlichia chaffeensis and Ehrlichia ewingii bacterial species
- Rocky Mountain spotted fever -- Dermacentor variabilis (American dog tick) and Rocky Mountain wood tick (Dermacentor andersoni) (hard tick) are the primary vectors and occasionally the brown dog tick (Rhipicephalus sanguineus); Amblyomma cajennense (hard tick) is the vector in countries south of the United States -- vectors for Rickettsia bacteria
- Southern tick-associated rash illness (STARI) -- Amblyomma americanum or lone star tick (hard tick) -- infectious agent not yet identified according to U.S. Centers for Disease Control and Prevention (CDC)
- Tick-borne relapsing fever -- Ornithodoros moubata or African tick (soft tick) -- vectors for Borrelia species of bacteria
- Tularemia -- Dermacentor variabilis (American dog tick) (hard tick) and Amblyomma americanum or lone star tick (hard tick) -- vectors for Francisella tularensis bacteria
- Anaplasmosis (human granulocytic anaplasmosis or HGA) -- Ixodes species (hard tick) -- vectors for Anaplasma phagocytophilum bacteria
- Colorado tick fever -- Dermacentor andersoni (hard tick) -- vectors for Coltivirus, a RNA virus
- Powassan encephalitis -- Ixodes species and Dermacentor andersoni (both hard ticks) -- vectors for Powassan encephalitis virus, an RNA arbovirus
- Q fever -- Rhipicephalus sanguineus, Dermacentor andersoni, and Amblyomma americanum (all three are hard ticks) -- vectors for Coxiella burnetii, a bacterium
Outbreaks of tick-related illnesses follow seasonal patterns (about April to September in the U.S.) as ticks evolve from larvae to adults. Different ticks go through complex life cycles that involve mating and larval formation and usually have several hosts; humans are usually not an essential part of the normal tick life cycle, but wherever a mammalian host is pictured in a tick life cycle, usually a human can replace the normal host animal. For example people could replace the deer or cow. However, in most cases, the life cycle is not completed with human hosts.