Infection, Infectious Diseases, and Epidemiology: Invasion and Establishment of Microbes

Infection, Infectious Diseases, and Epidemiology

Introduction

This section introduces the fundamental concepts of infection, infectious diseases, and the mechanisms by which microbes invade and establish themselves in the human body. Understanding these processes is essential for studying the pathogenesis of diseases and the principles of epidemiology.

Invasion and Establishment of Microbes: Infection

Exposure to Microbes: Contamination and Infection

• Contamination: Refers to the mere presence of microbes in or on the body. Not all contamination leads to infection or disease.

• Infection: Occurs when an organism evades the body's external defenses, multiplies, and becomes established in the body. Infection is a prerequisite for the development of infectious disease.

Portals of Entry

Pathogens must enter the host through specific sites known as portals of entry. These are anatomical sites where pathogens can gain access to host tissues.

• Skin: The outer layer of dead skin cells acts as a barrier to most pathogens. However, some pathogens can enter through cuts, abrasions, or by burrowing/digesting the outer layers.

• Mucous Membranes: These line body cavities open to the environment (e.g., respiratory, gastrointestinal, urogenital tracts). They provide a moist, warm environment favorable for pathogen entry. The respiratory tract is the most common portal of entry, with pathogens entering via the nose, mouth, or eyes. The gastrointestinal tract is another route, but pathogens must survive the acidic pH of the stomach.

• Placenta: Normally forms an effective barrier to most pathogens, but some can cross the placenta, potentially causing spontaneous abortion, birth defects, or premature birth.

• Parenteral Route: Not a true portal of entry, but refers to the direct deposition of pathogens into tissues beneath the skin or mucous membranes (e.g., via insect bites, punctures, or cuts), thereby circumventing the usual portals.

Examples of Portals of Entry

• Conjunctiva of the eye

• Nose

• Mouth

• Ear

• Broken skin

• Insect bite

• Placenta

• Vagina (in females), penis (in males)

• Urethra

• Anus

Table: Major Portals of Entry and Examples

Portal of Entry	Description	Examples
Skin	Barrier of dead cells; entry via cuts, abrasions, or burrowing	Staphylococcus aureus (via wounds), hookworm larvae (burrowing)
Mucous Membranes	Moist linings of body cavities; common entry for many pathogens	Influenza virus (respiratory tract), Salmonella (gastrointestinal tract)
Placenta	Barrier between maternal and fetal blood; some pathogens cross	Listeria monocytogenes, Treponema pallidum (syphilis)
Parenteral Route	Direct deposition into tissues beneath skin/mucosa	HIV (needle stick), Plasmodium (mosquito bite)

Key Points

• Not all exposure to microbes results in infection. The ability of a microbe to cause infection depends on its ability to overcome host defenses and establish itself.

• Portals of entry are critical in determining the pathogenesis of infectious diseases. Some pathogens are specific to certain portals, while others can use multiple routes.

Example

• Tetanus: Caused by Clostridium tetani, which typically enters the body through deep puncture wounds (parenteral route), bypassing the protective barrier of the skin.

• Influenza: The influenza virus enters primarily through the respiratory mucous membranes.