Plague circulates mainly among rodents, but the airborne(空気伝染する)bacteria can also infect humans. It can be effectively treated with antibiotics but is quite deadly without medication, capable of killing humans within 24 hours of infection. Pneumonic plague is caused by the same bacterium that causes bubonic plague(鼠蹊腺ペスト), an outbreak of which - now known as the "Black Death" - killed some 25 million people in Europe during the Middle Ages.