What is the definition of a pathogen?

The definition of a pathogen is specifically an organism that causes disease. Pathogens can include various types of microorganisms such as bacteria, viruses, fungi, and parasites. These organisms trigger harmful effects in their host and can disrupt normal bodily functions, leading to illness. Understanding this concept is crucial in the field of microbiology because it lays the foundation for studying how different organisms interact with the human body and what mechanisms are involved in the infection process.

In contrast, the other choices describe entities that do not fit the definition of a pathogen. An organism that provides health benefits is typically described as a probiotic or beneficial microbe, which plays a role in supporting health and normal bodily functions. A type of immune cell refers to specialized cells in the immune system, such as lymphocytes or macrophages, which are responsible for defending the body against pathogens and are not pathogens themselves. Lastly, a microorganism that is harmless to humans would be classified as non-pathogenic, which means it does not cause disease and can often coexist with human hosts without adverse effects.