Which term refers to disease-causing microorganisms such as viruses, bacteria, and parasites?

The term that refers to disease-causing microorganisms, including viruses, bacteria, and parasites, is "pathogens." Pathogens are entities that can lead to illness in a host organism. They actively invade cells or tissues and promote disease by various mechanisms, often leading to illness in the person or animal affected.

While allergens are substances that can cause allergic reactions and contaminants generally refer to unwanted substances that may pollute or spoil food, they do not specifically denote microorganisms. Toxins are poisonous substances produced by living organisms but do not encompass the broader category of disease-causing microbes. Therefore, "pathogens" accurately captures the essence of microorganisms that cause diseases.