Infectious Disease: Pathogens and Transmission

Module 7, Episode 1 focuses on Infectious Diseases.
Inquiry Question: How are diseases transmitted?
Syllabus reference: Classifying pathogens causing disease in plants and animals.

Disease: An abnormal structural or physiological condition that harms a living organism, lowering productivity or usefulness.
WHO definition of health: A state of complete physical, mental, and social well-being, not merely the absence of disease.

Non-infectious diseases:
- Caused by nutritional, environmental, genetics, and cancer factors.
- Non-communicable (cannot be caught).
Infectious diseases:
- Caused by pathogens (infectious agents).
- Communicable (contagious and can be spread).
- Examples: bacteria, viruses, and fungi.

Normal prion protein (PrPC): A naturally occurring protein found in cells, especially in the brain.
- Function is not fully understood but believed to play a role in cell signaling and protection against cellular stress.
Misfolded prion protein (PrPSC): An abnormal infectious form that induces further misfolding, leading to accumulation in the brain.
- Results in neurodegeneration and prion diseases.
- Generally considered incurable.
Examples of prion diseases:
- Creutzfeldt-Jakob disease
- Kuru
- Bovine Spongiform Encephalopathy (mad cow disease)
- Scrapie (in sheep and goats)

Microscopic infectious agents composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid).
Lack cellular structures; replicate inside host cells using the host's machinery.
Rapid replication and mutation make viral infections difficult to treat.
Example: COVID-19 with mutated spike proteins.
Treatment and Management:
- Some viruses can be treated with antiviral medications and vaccinations (e.g., HPV vaccine).
- Some viruses are incurable; medications manage symptoms (e.g., HIV, herpes simplex virus).
- Unmanaged HIV leads to AIDS.

Microscopic prokaryotic organisms; can be harmful or beneficial.
Infectious bacteria cause diseases in humans, animals, or plants.
Gram-positive bacteria:
- Retain crystal violet dye due to thick cell walls.
- Appear violet or blue under the microscope.
Gram-negative bacteria:
- Do not retain crystal violet dye due to thinner cell walls.
- Appear red or pink under the microscope.
Identification methods:
- Staining methods
- Shapes (cocci, bacilli)
Infectious bacteria invade host tissues, reproduce, and may produce toxins.
Treatment:
- Antibiotics
- Vaccinations (e.g., DTaP vaccine for diphtheria, tetanus, and pertussis).
Examples of bacterial diseases:
- Tuberculosis
- Cholera
- Strep throat
- Urinary tract infections
- STIs (chlamydia, syphilis)

Eukaryotic microorganisms with complex cell structures (nucleus and membrane-bound organelles).
Fungal infections can range from superficial skin infections to severe systemic diseases.
Examples:
- Ringworm (tinea corporis): contagious skin infection, red itchy ring-shaped rash.
- Athlete's foot (tinea pedis): affects feet, especially between toes.
- Thrush (mucocutaneous candidiasis): oral and vaginal.
- Systemic fungal infections: can invade the respiratory and nervous systems.
Treatment: Antifungal medication.
Prevention: Good hygiene practices.

Single-celled eukaryotic organisms (Protista kingdom).
Can be free-living or parasitic (leading to infection).
Pathogenic protozoa cause infectious diseases in animals and humans.
Example: Plasmodium (carried by Anopheles mosquito; causes malaria).
- Mosquito acts as a vector, transmitting Plasmodium into the bloodstream.
Other pathogenic protozoan diseases:
- Toxoplasmosis: transmitted from ingesting contaminated and undercooked meat, or from mother to baby during fetal development (congenital toxoplasmosis).