What is a microorganism that can cause disease in a susceptible host called?

A microorganism that can cause disease in a susceptible host is referred to as a pathogen. Pathogens are capable of inducing illness through various mechanisms, such as invading tissues, producing toxins, or evading the host's immune system. Common examples of pathogens include bacteria, viruses, fungi, and parasites. Understanding the role of pathogens is crucial in both medical and emergency settings, as recognizing and addressing infections is a fundamental part of patient care.

In contrast, an antigen is a substance that the immune system can recognize and respond to, typically part of pathogens but not necessarily disease-causing on its own. An allergen is a type of antigen that can cause allergic reactions in sensitive individuals, while a toxin refers to a poisonous substance produced by living organisms. Each of these terms has specific meanings within the context of immunology and infectious disease, distinguishing them from pathogens.