Publisher Summary The symptoms of leptospiral infection vary considerably in severity depending on which serovar causes infection. Leptospirosis is an acute, generalized infectious disease characterized by headache and fever, myalgia and chills, frequently with nausea but not always associated with vomiting and conjunctivitis. The muscular pain in the abdomen, the spinal region, and notably in the calf muscles is frequently severe and patients are usually prostrate. The onset is usually rapid and very frequently diagnosed as an influenza-like infection on initial presentation, particularly in nonendemic areas. This variable clinical outcome of leptospiral infection of humans is readily confused in the early presentation as a generalized influenza-like infection. The clinical manifestations of leptospiral infections vary while when isolates are compared, the physiological and biochemical characteristics in vitro are similar. The lack of sufficient phenotypic differences to support a realistic taxonomic scheme led inevitably to the construction of a differentiation scheme based primarily on serological features.