Bacillus anthracis, Clostridium botulinum, Vibrio Cholerae, Staphylococcus aureus and Mycobacterium tuberculosis are among the most harmful known bacteria, according to the National Academies of Sciences, Engineering and Medicine. These bacteria cause anthrax, botulism, cholera, staph infections and tuberculosis, respectively.
Anthrax commonly occurs in hoofed animals such as cows and sheep, explains the National Academies of Sciences, Engineering and Medicine. When this bacteria enters humans by eating contaminated meat, breathing the spores, or through a cut, it causes symptoms such as fever, chest pain, vomiting and abdominal pain. Once in the bloodstream, it may be fatal. Botulism typically comes from eating contaminated food, and causes difficulty swallowing and breathing, vomiting, double vision and possible paralysis. The bacteria can also enter the body through open sores or contact with untreated water.
Cholera is a deadly intestinal infection that causes more than 100,000 deaths each year, primarily due to dehydration that results from diarrhea, notes the National Academies of Sciences, Engineering and Medicine. As of 2015, a particular strain of staphylocuccus bacteria, called methicillin-resistant Staphylococcus aureus, or MRSA, has become a concern because the antibiotics traditionally used to treat these infections no longer work. Tuberculosis can also become resistant to antibiotics, but it is most prevalent in undeveloped countries, where these medicines aren't readily available. Tuberculosis commonly infects the lungs, but it can also affect other parts of the body such as the brain, kidneys and spine.
Possibly the most harmful bacteria in history is called Yersinia pestis, says Microbe World. This bacteria causes bubonic plague, which killed approximately 75,000,000 people in the 14th century.