Pathogens and Disease

What is a Pathogen?

A pathogen is a disease-causing organism. It enters the body, multiplies, and causes harmful effects.

Types of Pathogens

Transmissible Disease

A transmissible disease is one where a pathogen is passed from one host to another. This can spread within populations and cause epidemics or pandemics.

Transmission Methods

Direct Contact

• Physical contact between infected and healthy person
• Examples: kissing, sexual contact, touching infected wounds
• Pathogens transfer directly through skin or mucous membranes

Indirect Contact

Surfaces

• Touching contaminated objects (doorknobs, phones, utensils)
• Pathogens survive on surfaces and transfer to hands
• Then transferred to mouth, nose, or eyes

Food

• Contaminated food carrying pathogen
• Examples: undercooked meat, unpasteurized milk, contaminated produce
• Pathogen enters digestive system when food consumed

Animals

• Animals carrying pathogens to humans
• Examples: ticks carrying Lyme disease, mosquitoes carrying malaria
• Zoonotic diseases transmitted between animals and humans

Air

• Respiratory droplets from coughing, sneezing
• Aerosols containing pathogens remain airborne
• Examples: influenza, tuberculosis, measles
• Pathogen enters through respiratory tract

Body Defences

Physical Barriers

Skin

• Main defence: Complete physical barrier
• Prevents pathogen entry into body
• Dead outer layer (stratum corneum) prevents penetration
• Damaged skin (cuts, burns) provides entry points

Nose Hairs

• Trap particles and pathogens in inhaled air
• Prevent pathogens from reaching lungs
• Particles trapped can be expelled by blowing nose

Mucus

• Sticky substance lining respiratory and digestive tracts
• Traps pathogens and particles
• Contains antimicrobial enzymes (lysozyme)
• Helps expel pathogens through coughing, sneezing

Chemical Defences

Stomach Acid

• Hydrochloric acid in stomach (pH ~2)
• Kills many pathogens ingested with food
• Creates hostile environment for bacteria

Cellular Defences

White Blood Cells

• Cells in blood and tissues that fight infection
• Two main types:
  • Phagocytes: Engulf and destroy pathogens
  • Lymphocytes: Produce antibodies

Importance of Hygiene and Sanitation

Clean Water

• Prevents waterborne diseases (cholera, typhoid)
• Pathogens multiply in standing water
• Contaminated water spreads diseases easily

Hygienic Food

• Proper cooking kills pathogens
• Washing food removes contaminants
• Storage prevents bacterial growth

Personal Hygiene

• Hand washing: Removes pathogens from skin
• Covering mouth when coughing/sneezing
• Avoiding touching face with contaminated hands
• Not sharing personal items (utensils, towels)

Waste Disposal

• Proper disposal prevents pathogen spread
• Decomposing waste attracts disease-carrying insects
• Human waste contains pathogens in urine and feces

Sewage Treatment

• Treats wastewater before release
• Removes pathogens from sewage
• Prevents contamination of water sources
• Protects public health

Active Immunity

Antibody Production

Active immunity is defence by antibody production in response to pathogens.

Antigens

• Definition: Specific molecules on pathogen surface
• Each pathogen has unique antigens
• Immune system recognizes antigens as “foreign”

Antibodies

• Proteins produced by white blood cells (lymphocytes)
• Bind to antigens on pathogens
• Complementary shape to specific antigens (lock and key)
• Marked pathogens for destruction

How Antibodies Work

1. Recognition: Antibody binds to antigen on pathogen
2. Neutralization: Pathogen blocked from entering cells
3. Tagging: Pathogen marked for immune system
4. Destruction: Phagocytes engulf marked pathogens

Vaccination

How Active Immunity is Gained

Active immunity develops through two main routes:

Infection (Natural Immunity)

• Person contracts disease
• Body produces antibodies
• Provides immunity but requires suffering illness

Vaccination (Artificial Immunity)

• Person receives vaccine (safe form of pathogen)
• Body produces antibodies without illness
• Provides immunity without suffering disease

Vaccination Process

1. Antigen Introduction: Vaccine contains antigen (inactivated/attenuated pathogen or part of pathogen)
2. Lymphocyte Activation: Antigens stimulate lymphocytes
3. Antibody Production: Lymphocytes produce antibodies
4. Memory Cell Formation: Memory cells remain in body
5. Future Protection: On real infection, memory cells respond quickly

Memory Cells

• Specialized white blood cells
• Remember specific pathogens
• Persist for years or lifetime
• Respond rapidly on re-exposure
• Produce large amounts of antibodies quickly

Role of Vaccination in Controlling Disease

1. Individual protection: Vaccinated person cannot be infected
2. Herd immunity: High vaccination rates protect unvaccinated
3. Pathogen elimination: Widespread vaccination can eradicate diseases (smallpox)
4. Reduced spread: Less transmission in population

Passive Immunity

Short-Term Defence via Acquired Antibodies

Passive immunity receives pre-made antibodies rather than producing them.

Sources

Placenta

• Mother’s antibodies pass to fetus during pregnancy
• Provides temporary immunity to newborn
• Antibodies degrade over months

Breast Milk

• Contains maternal antibodies
• Provides immunity during early infancy
• Important when placental transfer insufficient
• Reduces infection risk in vulnerable period

Antibody Administration

• Medical treatment with antibodies (immunoglobulins)
• Used for immediate protection (rabies, tetanus)
• Temporary protection (weeks)

Importance of Breast-Feeding

1. Antibody protection: Babies receive maternal antibodies
2. Immune system development: Supports baby’s developing immunity
3. Nutrition: Provides nutrients and hydration
4. Reduced infections: Lowers risk of digestive/respiratory infections
5. Cost-effective: Free and accessible

Memory Cells in Passive Immunity

Memory cells are NOT produced in passive immunity.

• Only antibodies are transferred
• No immune memory developed
• Protection is temporary
• No long-term immunity

Cholera

Pathogen and Transmission

• Caused by bacterium: Vibrio cholerae
• Transmitted in water: Contaminated drinking water
• Water source contaminated with human faeces
• Pathogen enters digestive system

Cholera Toxin Effect

The cholera toxin causes severe symptoms through a specific mechanism:

1. Chloride ions secretion: Toxin damages intestinal cells
2. Chloride movement: Chloride ions move into gut lumen
3. Osmotic water movement: Water follows chloride ions by osmosis
4. Diarrhoea: Massive watery discharge from intestines
5. Dehydration: Rapid fluid and salt loss from body

Symptoms

• Severe watery diarrhoea (“rice-water stools”)
• Vomiting
• Rapid dehydration
• Muscle cramps
• Can be fatal if untreated

Treatment

• Rehydration: Replace lost fluids and salts
• Oral rehydration therapy: Salt and sugar solution
• Antibiotics: Reduce bacterial load
• Prevention: Clean water, sanitation, vaccination

Glossary