What is the term for a microorganism that causes an infection?

The term for a microorganism that causes an infection is "pathogen." Pathogens are biological agents that can lead to disease by invading a host and multiplying within it, resulting in infection. They can include a wide variety of organisms, such as bacteria, viruses, fungi, and parasites.

Prions, while they can cause disease, are misfolded proteins and not microorganisms in the traditional sense. Bioburden refers to the number and type of viable microorganisms present on a surface, typically in the context of sterilization and infection control, rather than being a specific organism that causes infection. Bacteria are a type of pathogen and can cause infections, but the broader term "pathogen" encompasses all types of infectious agents, including viruses and fungi, making it the most accurate choice.