Spirochetes are much more efficient than most bacteria at moving through very viscous media, which include the obstacles encountered when moving through an animal body.

Thus, both the spirochetes causing Lyme disease and those causing syphilis have high chances of reaching even the brain, while traveling through the body.

Fortunately, for the bacteria employing other means of locomotion it is usually more difficult to pass through many of the internal body tissues, as long as those are intact.