Tick-borne relapsing fever is caused by spirochetes within the genus Borrelia. The hallmark of this disease is recurrent febrile episodes and high spirochete densities in mammalian blood resulting from immune evasion. Between episodes of spirochetemia when bacterial densities are low, it is unknown whether ticks can acquire the spirochetes, become colonized by the bacteria, and subsequently transmit the bacteria once they feed again. We addressed these questions by feeding ticks, Omnithodoros hermsi Wheeler (Acari: Argasidae),daily on an infected mouse during low andhigh levels of spirochete infections. This study demonstrates that spirochete acquisition by the tick vector can occur during low levels of mammalian infection and that once a spirochetemic threshold is attained within the blood, nearly 100% of ticks become colonized by Borrelia hermsii.