C3.2.1—Pathogens as the Cause of Infectious Diseases

Three Major Types of Causes of Disease:

1. Pathogens: Microorganisms or agents that invade a host and cause disease. Examples include bacteria, viruses, fungi, protozoa, and parasites.
2. Genetic Factors: Diseases caused by genetic mutations or inherited conditions, such as cystic fibrosis or Down syndrome.
3. Environmental Factors: Diseases resulting from environmental exposure, such as pollution-related respiratory illnesses or diseases caused by exposure to harmful substances.

Pathogen Definition:

• Pathogen: An organism or substance that causes disease in another organism by disrupting normal physiological functions.

Major Pathogen Types:

1. Bacteria:
   • Structure: Prokaryotic cells, often with a cell wall, and can be rod-shaped (bacilli), spherical (cocci), or spiral (spirilla).
   • Examples: Streptococcus pneumoniae (pneumonia), Mycobacterium tuberculosis (tuberculosis).
2. Viruses:
   • Structure: Consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid). Some have an envelope derived from host membranes.
   • Examples: Human Immunodeficiency Virus (HIV), Influenza Virus.
3. Fungi:
   • Structure: Eukaryotic organisms with cell walls made of chitin. Can be unicellular (yeasts) or multicellular (molds).
   • Examples: Candida albicans (thrush), Aspergillus species (aspergillosis).
4. Protozoa:
   • Structure: Single-celled eukaryotes with complex cellular structures.
   • Examples: Plasmodium falciparum (malaria), Entamoeba histolytica (amoebiasis).
5. Parasites:
   • Structure: Varies widely; includes worms (helminths) and ectoparasites like lice.
   • Examples: Ascaris lumbricoides (giant roundworm), Plasmodium (malaria).

C3.2.2—Skin and Mucous Membranes as a Primary Defense

Primary Defense Definition:

• Primary Defense: The initial defense mechanisms that act to prevent pathogens from entering the body.

Roles in Defense:

1. Skin:
   • Barrier: Acts as a physical barrier preventing pathogen entry.
   • Antimicrobial Peptides: Produces antimicrobial peptides like defensins that inhibit microbial growth.
   • Acidity: Skin surface is slightly acidic, which inhibits pathogen growth.
2. Sebaceous Glands:
   • Secretion of Sebum: Sebum lubricates the skin and has antimicrobial properties, including fatty acids that deter microbial growth.
   • pH Maintenance: Helps maintain the skin’s slightly acidic pH, which is hostile to many pathogens.