Bacilli are a type of bacteria characterized by their rod-like shape. They belong to the broader category of prokaryotes, which are single-celled organisms without a nucleus. Bacilli can be found in various environments, including soil, water, and the human body, where some species play essential roles in processes like digestion. These bacteria can be beneficial or harmful. For example, Bacillus subtilis is a helpful bacterium used in fermentation and as a probiotic, while Bacillus anthracis is known for causing anthrax, a serious infectious disease. Understanding bacilli is important in fields like microbiology and medicine.