What is a pathogen?

A pathogen is defined as a microorganism that causes disease in its host. This includes bacteria, viruses, fungi, and protozoa that can invade an organism and lead to illness or dysfunction. In the context of aquatic pest control and plant health, understanding the role of pathogens is crucial as they can significantly impact the health of both aquatic ecosystems and terrestrial plants.

The other options provide definitions for different types of microorganisms that do not align with the definition of a pathogen. For instance, microorganisms that aid in plant growth or those considered beneficial for soil contribute positively to the plant's development and ecological balance rather than causing disease. Additionally, a virus that promotes plant health would also not be classified as a pathogen because it does not lead to disease but rather supports the health of the plant. Recognizing the specific characteristics of pathogens helps in developing effective management and control strategies for diseases they may cause.