Introduction

Disease- causing microorganisms are called pathogens.

Pathogenic microorganisms have special properties that allow them to invade the human body or produce toxins.

When a microorganism overcomes the body’s defenses, a state of disease results.

Pathology, Infection, and Disease

Pathology is the scientific study of disease.

Pathology is concerned with the etiology (cause), pathogenesis (development), and effects of disease.

Infection is the invasion and growth of pathogens in the body.

A host is an organism that shelters and supports the growth of pathogens.

Disease is an abnormal state in which part or all of the body is not properly adjusted or is incapable of performing normal functions (loss of homeostasis).

Normal Microbiota

Animals, including humans, are usually germ-free in utero.

Microorganisms begin colonization in and on the surface of the body soon after birth.

Microorganisms that establish permanent colonies inside or on the body without producing disease make up the normal microbiota.

Transient microbiota are microbes that are present for various periods and then disappear.

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(a) Surface of the skin	(b) Surface of the tongue	(c) Lining of the esophagus

Relationships Between the Normal Microbiota and the Host

The normal microbiota can prevent pathogens from causing an infection this phenomenon is known as microbial antagonism.

Normal microbiota and the host exist in symbiosis (living together).

There are three types of symbiosis:

Commensalism (one organism benefits and the other is unaffected)

Mutualism (both organisms benefit)

Parasitism (one organism benefits and one is harmed)

Opportunistic Microorganisms

Opportunistic pathogens do not cause disease under normal conditions but cause disease under special conditions.

Cooperation Among Microorganisms

In some situations, one microorganism makes it possible for another to cause a disease or produce more severe symptoms.