Infectious disease with recurring fever, caused by several spirochetes of the genus Borrelia, transmitted by lice, ticks, and bedbugs. Onset is sudden, with high fever, which breaks within a week with profuse sweating. Symptoms return about a week later. There may be 2 to 10 relapses, usually decreasing in severity. Mortality usually ranges from 0 to 6%, up to 30% in rare epidemics. Central nervous system involvement causes various (usually mild) neurological symptoms. The first microscopic organisms clearly associated with serious human disease (1867–68), the spirochetes mutate repeatedly, changing their antigens so that the host's immunity no longer is effective, which produces the relapses. Antibiotics can be effective, but inadequate therapy may leave spirochetes alive in the brain, and they may reinvade the bloodstream.
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