Causes of Disease Unit 1.1

Causes of Disease

Learning Objectives:

• What are pathogens?
• How do pathogens enter the body?
• How do pathogens cause disease?

Pathogens are microorganisms (general term for a single-celled organisms that is too small to see without the use of a microscope)  that cause disease. Disease is not a single thing but rather a description of certain symptoms, either physical or mental (or both). Disease suggests a malfunction of body or mind which has an adverse effect on good health. 

For a microorganism to be considered a pathogen it must:

• gain entry to the host
• colonise the tissues of the host
• resist the defences of the host
• cause damage to the host tissues.

Pathogens include bacteria, viruses and fungi. If a pathogen gets into the host and colonises its tissue an infection results. Disease occurs when an infection leads to recognisable symptoms in the host. When a pathogen is transferred from one individual to another it is known as transmission. 

A microorganism normally gets into the body by penetrating one of the organisms' interfaces (a surface or boundary linking…