Bacteria exist in a wide variety of forms and environments. Some are pathogens that cause disease in plants and animals, while others are harmless or even beneficial.

- Pathogenic bacteria are responsible for a variety of diseases, including pneumonia, tuberculosis, and food poisoning.

- Nonpathogenic bacteria do not cause disease and may actually be beneficial to the host organism. For example, certain bacteria help to break down food in the digestive system, while others help to produce vitamins and protect the body from infection.

- Commensal bacteria live on or in the body without causing any harm or benefit to the host organism.