Spores. Anthrax bacterial form spores with a thick coat and the spore form can remain dormant for years.
Spore. Anthrax can be found in soil through out the world. A hard coating like an armour forms to protect the anthrax bacteria. The spore is impervious to many types of attacks and it can remain dormant reportedly for hundreds of years in a dormant state. That means it can survive and infect at a later time.
Spores inhaled. The spores containing the bacteria bacillus anthracis can be inhaled into the lungs which then become very difficult to reach with antibiotics. Even worse if it spreads into the bloodstream.