Lyme disease (LD) is a frequent zoonosis found in the Northern Hemisphere and is considered an infectious disease caused by spirochetes belonging sensu lato to the Borrelia burgdorferi complex transmitted by ticks of the Ixodes ricinus group. In 1992, first cases similar to LD were described in Brazil, when brothers, after a tick bite episode developed symptoms , as erythema migrans, general flu-like symptoms and arthritis. Careful analysis of Brazilian LD-like illness casuistry showed that epidemiological, clinical and laboratorial features in the country were very different from those exhibited by North American and Eurasian LD patients. Human blood-suckers Ixodes ricinus complex ticks were absent at risk areas; the disease is recurrent in the country; Borrelia burgdorferi was never isolated in Brazil and specific serologic tests have shown little positivity with inconsistent results. Furthermore, peripheral blood analysis of patients on electron microscopy exhibited structures resembling Mycoplasma spp, Chlamydia spp and spirochete-like microorganisms. In fact, they were assumed to be latent forms of spirochetes (L form or cell wall deficient bacteria) adapted to survive at inhospitable conditions in vertebrate and invertebrate hosts. For these reasons, the Brazilian zoonosis was named Baggio-Yoshinari Syndrome (BYS) and defined as: "Exotic and emerging Brazilian infectious disease, transmitted by ticks not belonging to the Ixodes ricinus complex, caused by latent spirochetes with atypical morphology, which originates LD-like symptoms, except for occurrence of relapsing episodes and auto-immune disorders".