A biological agent that causes disease or illness to its host.

Here's why:

* Biological agent: Pathogens can be living organisms like bacteria, viruses, fungi, parasites, or even prions (misfolded proteins).

* Disease or illness: They cause a disruption to the normal functioning of the host organism, leading to symptoms and potentially serious health issues.

* Host: This is the living organism that the pathogen infects and where it replicates or multiplies.

Let me know if you'd like to know more about specific types of pathogens or how they cause disease!