Causal Agents:
There are many genera and species of ticks in the families Ixodidae (hard ticks) and Argasidae (soft ticks) that are of public health importance.  Some representative genera, and diseases they are known vectors for, include:  Amblyomma (tularemia, ehrlichiosis, Rocky Mountain spotted fever (RMSF), and boutonneuse fever); Dermacentor (RMSF, Colorado tick fever, tularemia, Siberian tick typhus, and Central European tick-borne encephalitis, as well as being an agent of tick paralysis); Hyalomma (Siberian tick typhus, Crimean-Congo hemorrhagic fever); Ixodes (Lyme disease, babesiosis, human granulocytic ehrlichiosis, and Russian spring-summer encephalitis); Rhipicephalus (RMSF and boutonneuse fever); Ornithodoros (tick-borne relapsing fever); Carios (tick-borne relapsing fever).

Life Cycle:
Most tick species undergo one of four different life cycles.  Members of the family Ixodidae undergo either one-host, two-host or three-host life cycles.  During the one-host life cycle, ticks remain on the same host for the larval, nymphal and adult stages, only leaving the host prior to laying eggs.  During the two-host life cycle, the tick molts from larva to nymph on the first host, but will leave the host between the nymphal and adult stages.  The second host may be the same individual as the first host, the same species, or even a second species.  Most ticks of public health importance undergo the three-host life cycle, whereby the tick leaves the host after the larval and nymphal stages.  The three hosts are not always the same species, but may be the same species, or even the same individual, depending on host availability for the tick.  Members of the family Argasidae undergo what is called a multihost life cycle.  Argasid ticks have two or more nymphal stages, each requiring a blood meal from a host.  Unlike the ixodid ticks, which stay attached to their hosts for up to several days while feeding, argasid ticks are adapted to feeding rapidly (about an hour) and then promptly leaving the host.

Geographic Distribution:
While ticks as a whole are worldwide in distribution, most species are restricted to various regions.  All major biogeographic regions (except Antarctica) have tick species of public health importance.