Definition of a pathogen and examples In the context of infectious disease, what can be considered a pathogen?

Introduction:
A pathogen is any biological agent capable of causing disease in a host. This question assesses basic infectious disease terminology and recognition of common pathogen types.

Given Data / Assumptions:

• Pathogens include diverse classes: viruses, bacteria, fungi, protozoa, helminths, and prions.
• “Agent that causes disease” is a generic definition.
• Visibility under a light microscope is not a criterion; many viruses are submicroscopic.

Concept / Approach:

Since both viruses and bacteria are well-established causes of disease, and a pathogen is by definition any disease-causing agent, a comprehensive option that encompasses all is correct.

Step-by-Step Solution:

Verification / Alternative check:

Textbook definitions of pathogenesis consistently categorize viruses and bacteria among primary pathogen types, alongside other microorganisms.

Why Other Options Are Wrong:

Option A, B, C: Each is partially correct but incomplete compared to the inclusive correct choice.

Option E: Size or light-microscopy visibility does not determine pathogenicity; viruses are not visible by standard light microscopy but are pathogens.

Common Pitfalls:

Assuming “pathogen” refers only to living cells; viruses and prions challenge that assumption.

Final Answer:

All of these