A spore is a reproductive cell produced by certain fungi, plants (moss, ferns), and some bacteria.

Certain bacteria make spores as a defensive mechanism. Bacterial spores have thick walls and are very resistant to high temperatures, humidity, and other unfavorable conditions. The bacteria Clostridia form spores that cause gas gangrene and antibiotic-associated colitis.

Chemical disinfection kills bacteria, but does not destroy their spores. Sterilization destroys spores as well as bacteria, and requires high temperatures and high pressures. In health-care settings, sterilization is usually accomplished using a device called an autoclave.