These diseases in humans are caused by infective organisms transmitted in the bite of a tick. The vector ticks vary with the geographical area: The usual transmitters are a wood tick Dermacentor andersoni in the western United States and a dog tick Dermacentor variabilis in the eastern United States. Other hard ticks transmit the causative agent in the southern United States and in Central and South America. Most human cases occur during late spring and summer.