Ticks are small arachnids in the order Parasitiformes. 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, Crimean Congo hemorrhagic fever, tularemia, tick-borne relapsing fever, babesiosis, ehrlichiosis, and tick-borne meningoencephalitis, as well as bovine anaplasmosis and probably the Heartland virus. Some species, notably the Australian paralysis tick, are also intrinsically venomous and can cause paralysis.