Female yellow dog tick, Amblyomma aureolatum, a vector of Rocky Mountain spotted fever (RMSF) in the country of Brazil. Note that the back of this tick is only partially covered by her hard, scutum, or "shield", which allows her abdomen to expand to many times the size seen here. Compare this feature with that of the male counterpart of this Ixodidae hard tick, PHIL 11070, and observe how the male's scutum covers his entire dorsal abdomen. Rocky Mountain spotted fever (RMSF) is the most severe tick-borne rickettsial illness in the United States. This disease is caused by infection with the bacterial organism Rickettsia rickettsii.