Bacteria are microscopic single-cell organisms that can live in just about any environment, including the body. They are single-celled with a simple internal structure that lacks a nucleus, but contains DNA. Some have one or more layers or protective outer covering; others have no cell wall at all.
Most bacteria multiply through binary fission, where a single bacterial cell makes a copy of its DNA and doubles it cellular content. The cell then splits in half and creates two identical cells. Others reproduce via budding, where a new cell grows as an offshoot of an existing cell. Once it’s the size of the original cell, it splits off.
The vast majority of bacteria do not cause diseases, and many are beneficial or even necessary for humans, for example they can aid in digestion. Others are detrimentally pathogenic, capable of causing infections and producing diseases by reproducing and crowding out healthy bacteria, or growing in tissues that are normally sterile. Antibiotics are typically used to treat bacterial infections. However, some strains of bacteria have become resistant to antibiotics making them difficult to treat. Examples of bacterial infections include: