Chapter 4: Human Health

4.1 Infectious and Non-Infectious Diseases

A disease is an abnormal condition that affects the body or mind, causing discomfort or dysfunction.

1. Infectious Diseases

Diseases that can be transmitted from one individual (host) to another. They are caused by pathogens (disease-causing microorganisms like bacteria, viruses, fungi, and protozoa) transferred through vectors or direct/indirect contact.

Transmission Medium	Examples of Diseases	Prevention / Control
Airborne	Tuberculosis (TB), Influenza, Measles, SARS, COVID-19	Cover mouth when sneezing/coughing, wear face masks, ensure adequate ventilation.
Waterborne	Cholera, Typhoid, Amoebic dysentery	Boil drinking water, add chlorine, maintain clean sanitation systems.
Direct Contact	Ringworm, Tinea (fungal infections), Syphilis, HIV/AIDS	Avoid sharing personal items (towels, combs), practice safe hygiene.
Vectors	Malaria (Anopheles mosquito), Dengue (Aedes mosquito), Leptospirosis (Rat)	Eliminate breeding grounds, use insecticides, keep living areas clean.

Common Vectors and Their Diseases

• Aedes Mosquito: Transmits Dengue virus and Zika virus.
• Anopheles Mosquito: Transmits Plasmodium protozoa (Malaria).
• Housefly: Transmits Vibrio cholerae (Cholera) and Salmonella typhi (Typhoid).
• Rat: Transmits Leptospira bacteria (Leptospirosis / Rat urine fever).

2. Non-Infectious Diseases

Diseases that cannot be passed from person to person. They are caused by genetic factors, lifestyle habits, environmental exposure, or aging.

• Examples: Cancer, Hypertension (High Blood Pressure), Diabetes Mellitus, Cardiovascular Diseases, Asthma.
• Prevention: Eating a balanced diet, exercising regularly, avoiding smoking and alcohol, managing stress.

4.2 Body Defence System

The human body possesses three lines of defence to protect against invading pathogens.

The Three Lines of Defence

1. First Line of Defence (Preventive - Non-specific):

   Prevents pathogens from entering the body.

   • Physical Barrier: Intact skin (sweat and sebum contain chemicals that destroy microbes).
   • Chemical Barrier: Mucus, saliva, tears (contain lysozyme enzyme), gastric juice (hydrochloric acid).
2. Second Line of Defence (Combat - Non-specific):

   Fights pathogens that manage to penetrate the first line of defence.

   • Phagocytosis: White blood cells (phagocytes) engulf and digest foreign pathogens using digestive enzymes.
   • Inflammation: Localized response causing swelling, heat, and redness to isolate and destroy pathogens.
3. Third Line of Defence (Targeted - Specific):

   Provides targeted immune responses against specific pathogens.

   • White blood cells (lymphocytes) produce antibodies that bind to foreign proteins (antigens) on pathogens, neutralizing or destroying them.

Types of Immunity

Immunity is the body's capability to resist disease through antibody production or cellular action.

Type of Immunity	Active Immunity (Body produces own antibodies)	Passive Immunity (Body receives ready-made antibodies)
Natural	Acquired after recovering from an infection (e.g., recovering from chickenpox).	Acquired by a baby through breast milk or placenta from the mother.
Artificial	Acquired through vaccination (injection of dead/weakened pathogens to stimulate antibody synthesis).	Acquired through antiserum / antiserum injection (injection of ready-made antibodies for immediate protection).

Importance of Immunisation

• Protects individuals and infants from fatal childhood diseases (e.g., Hepatitis B, Polio, Tetanus, Measles).
• Builds herd immunity in communities to safeguard vulnerable individuals who cannot be vaccinated.