Humans are the only natural reservoir of Treponema pallidum. It readily penetrates abraded skin and mucous membranes to multiply locally and spread through the blood stream or lymphatics. A developing maculopapular rash two weeks after a painless genital ulcer indicates a diagnosis of secondary syphillis. Ref: Primary care medicine: Office evaluation and management of the adult patient, by Allan H.Goroll,Albe G.Mulley, Page 989.