It is a bacillus, an organism, most usually carried by rodents. The flea then regurgitates the blood from the rat into the human, infecting the human. The rat dies. The human dies. The flea lives a long and happy life.
Nature has a morbid sense of humor.
Symptoms include high fevers and aching limbs and vomiting of blood. Most characteristic is a swelling of the lymph nodes. These glands can be found in the neck, armpits and groin. The swellings continue to expand until they eventually burst, with death following soon after. The whole process, from first symptoms of fever and aches, to final expiration, lasts only three or four days.