Pathogen Definition:
What Are Pathogens?

A pathogen is a microscopic organism who has two abilities:
(1) Cause sickness, and
(2) Spread from person to person.  In other words, they are “contagious“.

“Cause Sickness…and is contagious”

20160512c pathogen definition (color)
(click to enlarge)

Let us better understand the word “contagious“.  This word is an adjective that describes something that can spread from person to person.  Therefore, cancer is NOT contagious by this definition because it cannot spread from person to person.

What about diabetes?  Diabetes is a sickness that can be genetically passed down from parents to their children.  However, we do not call it contagious because a healthy person cannot “catch” diabetes like one catching a cold.  Instead, we call diabetes a “genetically inherited disease”, which means that it can be inherited from parents’ genes.

Next Generation Science Standards:

HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.

HS-LS1-3 Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis.

What Kind of Science Does this Comic Teach?

Unit: Anatomy

⇒Chapter: Immunology

⇒Topic: Pathogen Definition

Additional Information


A virus is the only pathogen that is NOT considered as a “living” organism.  This is because viruses’ physical construction is too simple to be considered as a living thing.  In fact, a virus is so simple it is not even considered as a cell, the simplest unit of living things!  As a result, viruses are not considered as “living organisms”.


To be called a pathogen, one must be “contagious”, meaning to be able to spread from person to person.


An organism that is so small that they are invisible to the naked eye and can only be viewed using certain instruments (such as microscopes).

Questions?  Comments?

For any questions, comments, or requests, contact us.


Please enter your comment!
Please enter your name here