Any of a variety of single-celled micro-organisms that can cause infectious diseases in humans and animals.

Examples of bacteria include streptococcus, which causes strep throat, and Helicobacter pylori (H. pylori), which causes inflammation and ulcers in the stomach and small intestine and is associated with an increased risk of stomach cancer.

Bacteria is the plural of bacterium. Bacterial means referring to or having to do with bacteria, as in bacterial infection.