What term is used to describe a harmful microorganism?

The term used to describe a harmful microorganism is "pathogen." Pathogens are biological agents that cause disease or illness in their hosts. This category includes a variety of microorganisms such as bacteria, viruses, fungi, and parasites that can lead to infections or other health issues.

When referring specifically to the options provided, while some microorganisms like bacteria and viruses can be classified as pathogens, the term "pathogen" encompasses all infectious agents known to cause harm, making it the most accurate and generalized term for harmful microorganisms.

Other terms like bacteria or virus refer to specific types of pathogens. Bacteria are a type of pathogen, while viruses are another type; however, neither term is as inclusive as "pathogen." Antigens, on the other hand, are substances that can induce an immune response but are not inherently harmful themselves. Therefore, "pathogen" is the correct and comprehensive term for describing harmful microorganisms.