The human flea is mainly a nuisance, an enzyme in their saliva producing an allergic reaction in their victims that causes an itching sensation. However, the parasite can also be a vector of a variety of diseases. Though it was not the primary species responsible for spreading the bubonic plague throughout Europe during the Middle Ages, the human flea is capable of transmitting it. The species may also transmit murine typhus, tularemia, and tapeworm, but such occurrences are relatively rare.