Human Health: Infectious Diseases, Transmission Vectors, Body Defence Systems, and Immunisation

Classification of Diseases and Pathogenic Microorganisms

  • Diseases are broadly classified into two categories:
    • Infectious Diseases: Diseases that can be transmitted from one person to another.
    • Examples include cholera, flu, tuberculosis (TB), ringworm, tinea, dengue fever, malaria fever, and leptospirosis.
    • Non-infectious Diseases: Diseases that cannot be transmitted from one person to another.
    • Examples include cancer, diabetes, hypertension, heart disease, cardiovascular disease, and asthma.
  • Pathogens: Harmful microorganisms that cause infectious diseases.
  • Infectious diseases are spread by pathogens transmitted from an infected person (host) to another person through various routes, including air, water, contact, and vectors.
  • Microorganisms that cause diseases include bacteria, viruses, fungi, and protozoa.
    • Bacteria: Cause tuberculosis (TB), cholera, gonorrhea, and syphilis.
    • Viruses: Cause flu, influenza A (H1N1), dengue fever, zika fever, acquired immunodeficiency syndrome (AIDS), and chickenpox.
    • Fungi: Cause ringworm and tinea.
    • Protozoa: Cause malaria fever and dysentery.

Classification of diseases, airborne transmission, and waterborne transmission overview

Transmission Routes and Preventive Measures

  • Airborne Diseases:

    • Pathogens are transmitted through the air in two primary ways:
    • Droplet Transmission: Tiny water droplets containing pathogens are expelled from the mouth and nose when an infected person sneezes, coughs, talks, yawns, or breathes. These droplets are carried away by air and inhaled by healthy individuals. Infections spread rapidly in crowded areas with moist air.
    • Dust Transmission: Pathogens contained in the dried saliva of an infected person form spores, which are dispersed together with dust by the wind. Inhalation of this airborne dust causes infection.
    • Specific Examples of Airborne Transmission:
    • Measles virus spreads when rashes on the skin of an infected person dry up and are carried away with dust by the wind.
    • Other airborne diseases include flu, tuberculosis (TB), influenza A (H1N1), and Severe Acute Respiratory Syndrome (SARS).
    • Prevention of Airborne Diseases:
    • Cover the mouth when sneezing, coughing, and yawning.
    • Do not spit indiscriminately.
    • Avoid staying in crowded areas.
    • Quarantine infected patients to prevent transmission.

  • Waterborne Diseases:

    • Infection occurs when natural or municipal water sources such as wells, ponds, lakes, and rivers become contaminated with pathogens.
    • Outbreaks typically happen in regions with inadequate treated water supplies and poor sanitation.
    • In unsanitary areas, toilets constructed near riverbanks discharge faeces directly into river systems, polluting the water with pathogens.
    • Residents contract diseases by drinking or using this contaminated water.
    • Flooding also acts as a major transmission vehicle for waterborne infectious diseases.
    • Examples of waterborne diseases include cholera, typhoid, and dysentery.
    • Prevention of Waterborne Diseases:
    • Drink only boiled water.
    • Add chlorine into swimming pools and municipal water supply systems.
    • Construct toilets equipped with proper sanitary systems.
    • Wash hands thoroughly with soap after using the toilet and prior to eating.

  • Infection through Contact:

    • Diseases are transmitted via indirect contact or direct contact.
    • Indirect Contact:
    • Pathogens spread when sharing personal items such as towels, clothing, and combs with infected individuals.
    • Examples include fungal infections such as ringworm and tinea.
    • Direct Contact:
    • Transmission occurs through direct physical exchange with infected persons via sexual intercourse, blood transfusion, and the sharing of injection needles.
    • Sexually transmitted diseases (STDs) such as syphilis and gonorrhea, as well as AIDS and hepatitis B, are transferred through semen and vaginal fluids during sexual intercourse.
    • AIDS is also transmitted through contaminated blood transfusions and needle sharing among intravenous drug addicts.
    • Prevention of Contact Transmission:
    • Maintain personal hygiene, family cleanliness, and home sanitation.
    • Practice a healthy lifestyle.
    • Avoid sharing personal items such as towels, combs, and clothing.
    • Maintain physical distance and stay away from infected individuals.

Vectors, pathogens, and methods of mosquito and fly control

  • Infection through Vectors:
    • Vectors: Organisms that carry pathogens and transfer them from one host to another (e.g., mosquitoes, flies, cockroaches, and rats).
    • Mechanisms of Vector Transmission:
    • Mosquito Bites: A mosquito acquires pathogens when biting an infected individual. The pathogens migrate to and enter the mosquito\'s salivary glands. When biting a healthy host, the mosquito injects saliva to prevent blood clotting; through this saliva, pathogens enter the host\'s bloodstream.
      • Diseases transmitted include dengue fever, zika fever, and malaria fever.
    • Food Contamination by Flies: Flies landing on the faeces of infected individuals pick up pathogens on their legs and bodies. When landing on food, they deposit these pathogens onto the food surface.
      • Diseases transmitted include cholera and dysentery.
    • Pathogens and Their Associated Vectors:
    • Aedes mosquito: Transmits Dengue virus and Zika virus.
    • Housefly: Transmits Salmonella typhi (bacteria).
    • Cockroach: Transmits Salmonella typhi (bacteria).
    • Anopheles mosquito: Transmits Plasmodium malariae (protozoa).
    • Rat: Transmits Leptospira sp. (bacteria).
    • Vector Control and Prevention Strategies:
    • Vector Control Objective: Eradicate organisms that carry and transmit pathogens using knowledge of their habits and life cycles.
    • Mosquito Population Control (Targeting Larvae and Pupae): Female mosquitoes deposit eggs in stagnant water.
      • Ensure no open containers or objects collect standing water in the vicinity.
      • Spray a layer of oil onto the surface of ponds or drains to prevent larvae and pupae from accessing atmospheric oxygen.
      • Introduce biological predators, such as guppy fish, into ponds to feed on mosquito larvae.
      • Add insecticides or salt into stagnant water reservoirs (e.g., water storage tanks, clay jars, flower pots) to destroy larvae.
    • Prevention of Mosquito Bites:
      • Spray insecticides.
      • Fit windows with fine mesh netting.
      • Apply mosquito repellent to skin and clothing.
      • Sleep beneath protective mosquito nets.
    • Fly Population Control: Female flies lay eggs on exposed food or decaying organic food waste.
      • Cover all food items to prevent flies from landing.
      • Dispose of food waste into dustbins fitted with tightly closed lids.
      • Maintain rigorous cleanliness inside the house and around outdoor premises to eliminate breeding grounds.
      • Eliminate adult houseflies using fly tape, electric fly traps, and insecticides.

Summary of Infectious Diseases, Pathogens, Transmission, and Symptoms

Infectious diseases summary table, stages of disease prevention, and lines of body defence

  • Tuberculosis (TB):
    • Pathogen: Bacteria.
    • Transmission: Air or water droplets expelled via coughing and dried phlegm.
    • Symptoms: Fatigue, persistent cough, coughing up blood, fever, and weight loss.
  • Cholera:
    • Pathogen: Bacteria (Vibrio cholera).
    • Transmission: Ingestion of contaminated food and water.
    • Symptoms: Severe diarrhoea, vomiting, and extreme dehydration.
  • Typhoid:
    • Pathogen: Bacteria (Salmonella typhi).
    • Transmission: Ingestion of contaminated food and water.
    • Symptoms: High fever, intestinal bleeding, and red skin rashes.
  • Leptospirosis:
    • Pathogen: Bacteria (Salmonella typhi / Leptospira sp.).
    • Transmission: Exposure to contaminated soil, food, and water.
    • Symptoms: Fever, severe headache, and muscle pain.
  • Flu:
    • Pathogen: Virus.
    • Transmission: Air and water droplets.
    • Symptoms: Fever, headache, sneezing, sore throat, and runny nose.
  • Dengue Fever:
    • Pathogen: Virus.
    • Transmission: Bites from infected female Aedes mosquitoes.
    • Symptoms: High fever, headache, severe joint pain, watery eyes, and red spots across the skin.
  • Zika Fever:
    • Pathogen: Virus.
    • Transmission: Bites from infected female Aedes mosquitoes.
    • Symptoms: Fever, cutaneous rashes, joint pain, and conjunctivitis (red eyes).
  • Acquired Immunodeficiency Syndrome (AIDS):
    • Pathogen: Human Immunodeficiency Virus (HIV).
    • Transmission: Blood transfusions, sharing of contaminated injection needles, and unprotected sexual intercourse.
    • Symptoms: Recurrent fever, rapid weight loss, chronic severe diarrhoea, and increased susceptibility to opportunistic infections due to severe immune system failure.
  • Malaria Fever:
    • Pathogen: Protozoa (Plasmodium malariae).
    • Transmission: Bites from infected female Anopheles mosquitoes.
    • Symptoms: Episodes of high fever followed by severe shivering and chills.
  • Ringworm:
    • Pathogen: Fungi.
    • Transmission: Direct skin contact with patients or sharing personal items.
    • Symptoms: Circular, itchy red rashes on the skin.
  • Tinea:
    • Pathogen: Fungi.
    • Transmission: Direct skin contact with patients or sharing personal items.
    • Symptoms: Itchy white discoloured patches or spots on the skin.

Stages of Prevention Mechanisms

  • Primary Stage:
    • Focuses on general prophylaxis and health enhancement prior to exposure.
    • Maintain rigorous personal hygiene and environmental cleanliness.
    • Fortify the internal body defence system by receiving scheduled vaccine injections.
    • Attend frequent routine health check-ups.
    • Practice a healthy lifestyle and consume a nutritionally balanced diet.
  • Secondary Stage:
    • Focuses on interrupting and disconnecting active disease transmission chains.
    • Administer early clinical treatment to diagnosed patients.
    • Enforce quarantine procedures on infected individuals to halt transmission to the public.
  • Tertiary Stage:
    • Focuses on controlling vector reservoirs and protecting susceptible host populations.
    • Suppress and eradicate the population of biological vectors.
    • Shield hosts from infection using physical barriers, including mosquito bed nets, mosquito repellent coils, and thick clothing.

Human Body Defence Mechanisms

  • The human body employs defence systems to identify, restrict, and neutralize invading pathogens.
  • The body defence mechanism is organized into three lines of defence:
    • Non-specific Body Defence Mechanisms: Consist of the First Line of Defence and the Second Line of Defence, which act non-selectively against all foreign invaders.
    • Specific Body Defence Mechanism: Consists of the Third Line of Defence, which responds specifically to distinct foreign antigens.

Phagocytosis process, mechanism of antibody action, and principles of immunisation

  • First Line of Defence:

    • Acts as an external physical and chemical barrier preventing all types of microorganisms from penetrating outer body tissues.
    • Skin:
    • Possesses an outermost stratum of dead cells in the epidermis that forms an impenetrable physical barrier.
    • Sweat secreted by sweat glands contains antimicrobial chemicals that destroy bacteria, along with acidic compounds that suppress bacterial growth.
    • Sebum is a waterproof lipid secretion that forms a protective film over the skin surface to block microorganism entry; it also contains chemical substances capable of killing microorganisms.
    • Mucous Membranes:
    • Line the respiratory tract, urinary tract, and reproductive system.
    • Secrete sticky mucus that physically traps and chemically destroys microorganisms.
    • Nasal Hair: Fine hairs lining the nasal passages filter out suspended microorganisms and dust particles from inhaled air.
    • Cilia: Microscopic, hair-like projections situated along the upper respiratory tract that trap foreign particles and sweep mucus containing pathogens upwards toward the throat to be expelled or swallowed.
    • Lysozyme Enzyme: Present in tears, sweat, and saliva; functions as an antimicrobial enzyme that hydrolyses and kills certain bacterial cell walls.
    • Gastric Juice: Highly acidic stomach fluid with a pH of pH 2−3\text{pH } 2 - 3; its high acidity destroys ingested bacteria and denatures microbial toxins.
    • Earwax: Secreted within the ear canals to physically trap dust and protect the auditory canal from bacterial invasion.
    • Excretion and Expulsion: Pathogens are continuously eliminated from the body via urine, faeces, and vomiting.

  • Second Line of Defence:

    • Initiated when pathogens successfully breach the first line of defence and enter internal tissues or the bloodstream.
    • Mediated by white blood cells that engulf and destroy pathogens through phagocytosis.
    • Phagocytosis Process:
    1. A circulating white blood cell detects, approaches, and surrounds the invading pathogen.
    2. The white blood cell extends its membrane to engulf the pathogen, enclosing it inside an internal cellular vacuole known as a phagosome.
    3. A cellular lysosome fuses with the phagosome and secretes the digestive enzyme lysozyme, breaking down and digesting the pathogen completely.

  • Third Line of Defence:

    • Activated when pathogens evade non-specific defences, relying on the immune system to synthesize specific antibodies.
    • Antibody: A specialized functional protein synthesized and secreted into the bloodstream by white blood cells that binds specifically to an invading foreign antigen.
    • Antigen: A foreign macromolecule or particulate substance originating outside the body that stimulates an immune response.
    • Antigens are located on the outer surface of pathogens, on toxic molecular compounds produced by bacteria or snake venom, and on foreign red blood cells from incompatible blood groups.
    • Immunity: The biological ability of the bodily systems to resist pathogen proliferation and overcome disease prior to infection taking hold.
    • Mechanisms of Antibody Action:
    • Agglutination / Clumping: Antibodies bind directly to surface antigens on multiple pathogens simultaneously, cross-linking them into clumps that render the pathogens dysfunctional and easier for phagocytes to destroy.
    • Neutralisation: Specific antibodies combine directly with toxic protein molecules, neutralizing their active sites and rendering the toxins harmless.

Active and Passive Immunity

Classification tree map of immunisation and antibody concentration response graphs

  • Immunisation:

    • The medical process of establishing immunity or resistant capacity in an individual against specific infectious diseases by administering vaccines.
    • Vaccine: A biological preparation containing antigens derived from dead, attenuated (weakened), or fragmented bacteria or viruses.
    • When injected into the body, vaccines stimulate white blood cells to produce specific antibodies and develop immunological memory without inducing full-blown disease.
    • Medical Importance:
    • Protects infants and children against life-threatening diseases that cause physical deformity or fatality.
    • Aligns with the medical principle that prevention is superior to cure.
    • Halts the emergence and spread of widespread epidemics.
    • Must be administered according to precise age schedules to achieve maximum efficacy and enduring protection.

  • Classification of Immunity:

    • Immunity is divided into Active Immunity and Passive Immunity, both of which exist in Natural and Artificial forms.

  • Active Immunity (Body Actively Produces Antibodies):

    • Natural Active Immunity:
    • Acquired naturally after recovering from an infectious disease.
    • During initial exposure, antibodies are produced slowly, but immunological memory cells remain in the circulation.
    • Upon a secondary infection by the same pathogen, the immune system rapidly produces large quantities of specific antibodies, quickly exceeding the critical immunity level and suppressing the disease before clinical symptoms develop.
    • Confers long-lasting or lifelong immunity against specific diseases such as chickenpox and mumps.
    • Artificial Active Immunity (Vaccination):
    • Acquired artificially through the injection of vaccines.
    • The initial vaccine dose induces antibody synthesis, but the antibody concentration in the blood remains below the required immunity level.
    • A secondary booster injection is required to stimulate a rapid, amplified production of antibodies that surpasses the threshold immunity level, ensuring long-term protection.
    • Administered routinely against diseases such as tuberculosis (TB), rubella, and poliomyelitis.

  • Passive Immunity (Body Passively Receives Pre-formed Antibodies):

    • Natural Passive Immunity:
    • Acquired naturally without initiating an active immune response by the recipient.
    • Occurs when a developing foetus absorbs maternal antibodies across the placenta, or when a newborn baby ingests maternal antibodies present in breast milk (colostrum and milk).
    • Provides temporary protection that degrades over time, lasting only a few months after birth.
    • Confers short-term protection to newborns against conditions such as common fever and pneumonia.
    • Artificial Passive Immunity (Antiserum):
    • Acquired artificially through the intramuscular or intravenous injection of antiserum.
    • Antiserum: Blood serum separated from an immunized donor animal or individual that contains concentrated, pre-formed antibodies against a specific pathogen or toxin.
    • Operates without activating the patient\'s internal immune machinery.
    • Action Profile:
      • Immediate Action: Therapeutic protection is instantaneous, making it suitable for treating severely ill, immunocompromised, or acutely poisoned patients.
      • Non-Permanent Action: Because the recipient does not generate their own memory cells, the transferred antibodies degrade, causing blood antibody concentrations to drop below the immunity level within weeks. Repeated doses are necessary during sustained exposure.
    • Clinically deployed as emergency anti-toxins and treatments for tetanus, rabies, diphtheria, and venomous snake bites.

  • Key Terminology:

    • Antigen: Antigen
    • Immunity: Keimunan
    • Immunisation: Imunisasi