What is a pathogen?

A pathogen is defined as a microorganism that causes disease. This includes bacteria, viruses, fungi, and parasites that can invade a host organism, multiply, and disrupt normal physiological function, leading to illness. The focus on disease causation emphasizes the ability of these microorganisms to elicit harmful effects in their hosts, which distinguishes them from other types of microbes that may not cause disease or may even be beneficial.

Beneficial bacteria, for instance, play crucial roles in processes such as digestion and nutrient absorption and do not typically lead to disease. Antibiotic-resistant strains represent a specific challenge in treating infections but are not inherently classified as pathogens unless they also cause disease. Similarly, toxic substances produced by bacteria, known as toxins, can contribute to disease but are not themselves classified as pathogens. Thus, the definition and role of pathogens are specifically tied to their capability to cause illness in hosts.