This form is an active spiral microbe which is found abundantly in the blood of persons affected with relapsing fever. (See Fig. 148.) It is present only during the acute attacks, disappearing in the interval, to return when the relapse occurs. It is readily stained by ordinary watery solutions of the aniline dyes but not by Gram's method. The disease has been communicated to man and to apes by the inoculation of blood containing the spirilla. The spirillum is found only in the blood, and Koch has observed it in the blood of the brain, liver, and kidneys of an ape which was killed during the attack. These spirilla have not been cultivated artificially.