pustule : Idioms & Phrases

• noun a form of anthrax infection that begins as papule that becomes a vesicle and breaks with a discharge of toxins; symptoms of septicemia are severe with vomiting and high fever and profuse sweating; the infection is often fatal
  cutaneous anthrax.
  

WordNet

• (Med.), a very contagious disease, transmitted to man from animals, characterized by the formation, at the point of reception of the virus, of a vesicle or pustule which first enlarges and then breaks down into an unhealthy ulcer. It is marked by profound exhaustion and usually fatal. Called also charbon, and sometimes, improperly, anthrax.

Webster 1913