Detailed Solution:

The correct answer is “Pathogens”.

Pathogens are microorganisms that cause diseases in humans, animals, and plants. They include bacteria, viruses, fungi, and protozoa.

Bacteria can be pathogenic (disease-causing) or harmless/helpful (e.g., gut bacteria).

Protozoa are single-celled organisms, some of which (like Plasmodium causing malaria) are pathogens.

Archaea are a distinct group of microorganisms that do not cause diseases in humans.