Spirochetal infection of the central nervous system. Four spirochetal diseases frequently involve the central nervous system: syphilis, leptospirosis, relapsing fever, and Lyme borreliosis. In particular, syphilis and Lyme borreliosis are increasing problems. During the spirochetemic phase there is seeding of the nervous system. After a quiescent latent period, there may be late disease flareups producing a variety of neurologic syndromes. Cerebrospinal fluid examination is very helpful in these infections. 