Ticks are small arachnids in the order Ixodida. Along with mites, they constitute the subclass Acarina. Ticks are ectoparasites (external parasites), living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians. Ticks are vectors of a number of diseases, including Lyme disease, Q fever (rare; more commonly transmitted by infected excreta), Colorado tick fever, Rocky Mountain spotted fever, African tick bite fever, tularemia, tick-borne relapsing fever, babesiosis, ehrlichiosis, Tick paralysis and tick-borne meningoencephalitis, as well as bovine anaplasmosis. There are three families of ticks, one of which Nuttalliellidae comprises a single species, Nuttalliella namaqua. The remaining two families contain the hard ticks (Ixodidae) and the soft ticks (Argasidae). Ixodidae are distinguished from the Argasidae by the presence of a scutum or hard shield.