Three main species (Borrelia garinii, Borrelia afzelii, and B burgdorferi s.s.) are the main causative agents of the disease in humans, while a number of others have been implicated as possibly pathogenic. Borrelia species known to cause Lyme disease are collectively called Borrelia burgdorferi sensu lato (s.l.). Borrelia are microaerophilic and slow-growing—the primary reason for the long delays when diagnosing Lyme disease—and have been found to have greater strain diversity than previously estimated.