What characterizes a pathogen?

A pathogen is characterized as a bacterium, virus, or other microorganism that can cause disease in a host. The defining feature of a pathogen is its ability to disrupt normal physiological functions and contribute to illness, either through direct damage to host tissues, the release of toxins, or triggering harmful immune responses. Pathogens can include a wide range of organisms, such as bacteria like Streptococcus or viruses like influenza, all of which have mechanisms to invade host tissues and overcome immune defenses.

In contrast, a virus that benefits both the host and itself would not be classified as a pathogen since it implies a symbiotic relationship where the host is not harmed, and hence no disease is caused. Similarly, a microorganism that does not cause disease clearly falls outside of the definition of a pathogen as it does not fulfill the criteria of causing harm to the host. Finally, organisms that thrive on decaying organic matter, such as decomposers, play an essential role in nutrient cycling and do not typically engage in pathogenic interactions with living hosts. Therefore, the emphasis on the potential of certain microorganisms to cause disease is what makes option C the correct characterization of a pathogen.