The causative organism is "Rickettsia prowazekii", transmitted by the human body louse ("Pediculus humanus corporis"). Curiously, this pattern is not found in the human body louse ("Pediculus humanus"). Despite this, the modern view is that only the body louse can transmit the disease. Other lice that infest humans are the body louse and the crab louse. For example, the human body louse transmits the bacterium "Rickettsia prowazekii" which causes epidemic typhus. If the conditions are met, the body louse can reproduce rapidly in favoring conditions. Since the 1800s, the body louse has been known as its only known vector. The body louse belongs to the Phylum Arthropoda, Class Insecta, Order Phthiraptera and Family Pediculidae. The human body louse "Pediculus humanus humanus" has (2010) the smallest insect genome known.