Which of the definitions below best defines a pathogen?

A change in a cell's genetic material

An infectious agent that causes disease

A substance that increases in concentration

The influence of human beings on nature

It's an infectious agent that causes disease

Option (b).

Explanation:

Pathogens may be defined as the organism that have the ability to cause disease in the organism. The organism that can acts as pathogens are bacteria, virus, fungus.

Pathogens has the ability to evade the immune response of the host organism. The pathogens release specific chemicals or substances that cause disease in the host organism.

Thus, the correct answer is option (b).