Final answer:
Anthrax, caused by the bacterium Bacillus anthracis, is classified as a biological weapon. It was used in a bioterrorism attack in the United States in 2001, where it caused infections and deaths through mailed letters containing its spores.
Step-by-step explanation:
The disease-causing bacteria known as anthrax are classified as biological weapons. Anthrax is caused by the bacterium Bacillus anthracis, which is a serious threat due to its potential for mass harm when spores are aerosolized and inhaled. This microorganism can lead to deadly infections if not treated promptly. In 2001, anthrax spores were used in an act of bioterrorism in the United States, where letters containing the spores were sent via mail, resulting in multiple infections and deaths. Considering the nature and use of anthrax, it fits into the category of biological weapons rather than chemical or conventional weapons. Therefore, the correct answer to the question 'Disease-eating bacteria called anthrax are known as ___________.' is D. biological weapons.