Bacteria infecting a macrophage. Coloured scanning electron micrograph (SEM) of Bacillus anthracis bacteria (pink) infecting a macrophage white blood cell. The macrophage, when activated, will engulf (phagocytose) the bacteria and destroy them as part of the body's immune response. B. anthracis is a Gram-positive, spore-forming bacteria that causes anthrax. It can infect the skin (cutaneous anthrax), causing raised itchy lesions, the lungs (pulmonary anthrax), which is fatal unless treated quickly, and the digestive system (gastrointestinal anthrax), causing vomiting of blood and severe diarrhoea. All forms can be fatal if left untreated. Treatment is with antibiotics.