Human Health and Disease

Human Health and Disease

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

• Health is a state of complete physical, mental, and social well-being, not merely the absence of disease (WHO definition).
• Pathogens are disease-causing organisms: bacteria (e.g. Mycobacterium tuberculosis, Vibrio cholerae), viruses (HIV, measles, polio, influenza), protozoans (Plasmodium in malaria, Trypanosoma in sleeping sickness), fungi (ringworm), and rickettsias.
• Vectors carry pathogens: Anopheles mosquito → malaria; tsetse fly → trypanosomiasis; Aedes mosquito → yellow fever; housefly → cholera/typhoid.
• The body defends itself in three lines: skin/mucous membranes → phagocytes and inflammation → specific immunity via B-lymphocytes (antibody production) and T-lymphocytes (cell-mediated killing).
• Vaccination stimulates antibody production artificially without causing disease — the basis of immunisation programmes tested in NABTEB.
• Drug abuse consequences: tobacco → lung cancer/emphysema; alcohol → liver cirrhosis; hallucinogens/stimulants → nervous system damage.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Classification of Diseases

Diseases fall into two broad groups. Communicable (infectious) diseases spread from one person (or animal) to another via pathogens — examples are tuberculosis, malaria, cholera, HIV/AIDS, and measles. Non-communicable diseases do not spread between individuals; they include degenerative disorders (e.g. hypertension), deficiency diseases (e.g. scurvy from lack of vitamin C), hereditary disorders (e.g. sickle cell anaemia), and mental/social illnesses (e.g. depression).

Common Pathogens and the Illnesses They Cause

• Bacteria: Mycobacterium tuberculosis → tuberculosis (lungs); Vibrio cholerae → cholera (severe watery diarrhoea); Salmonella typhi → typhoid fever; Neisseria gonorrhoeae → gonorrhoea.
• Viruses: HIV (a retrovirus) → AIDS; poliovirus → poliomyelitis; rubeola virus → measles; influenza viruses → flu; SARS-CoV-2 → COVID-19.
• Protozoa: Plasmodium falciparum/vivax → malaria; Trypanosoma brucei → sleeping sickness (trypanosomiasis); Entamoeba histolytica → amoebic dysentery.
• Fungi: Trichophyton → ringworm (tinea); Candida albicans → thrush.
• Rickettsias (obligate intracellular parasites): cause typhus, transmitted by lice/fleas.

Transmission Routes and Vectors

Biological vectors are living organisms that transmit pathogens: Anopheles mosquito injects Plasmodium during a blood meal; tsetse fly (Glossina) transmits Trypanosoma; Aedes mosquito spreads yellow fever virus. Mechanical vectors like the housefly carry pathogens on their bodies from faeces to food. Other routes include airborne droplets (tuberculosis, influenza), contaminated water (cholera, typhoid), sexual contact (HIV, gonorrhoea), and blood transfusion.

The Three Lines of Body Defence

1. First line (non-specific barriers): skin, mucous membranes, tears (containing lysozyme), saliva, and stomach acid (HCl) physically or chemically block entry of pathogens.
2. Second line (non-specific cellular response): phagocytes (neutrophils, macrophages) engulf and digest pathogens; inflammation increases blood flow and recruits immune cells; fever slows pathogen replication.
3. Third line (specific/adaptive immunity): B-lymphocytes produce antibodies that target specific antigens; T-lymphocytes directly destroy infected body cells. Memory cells remain after infection, providing long-term immunity.

Immunity and Vaccination

Active immunity arises when the body itself produces antibodies (natural: after infection; artificial: after vaccination). Passive immunity is gained by receiving ready-made antibodies (natural: from mother via placenta or breast milk; artificial: antitoxin injection). Vaccination introduces a weakened, killed, or subunit form of the pathogen, triggering B-cell antibody production without causing the disease — the foundation of immunisation schedules tested in NABTEB.

Drug Abuse and Public Health

Tobacco smoking causes lung cancer, chronic bronchitis, and emphysema (destruction of alveolar walls). Chronic alcohol consumption leads to liver cirrhosis, brain damage, and addiction. Hallucinogens (LSD) and stimulants (cocaine, amphetamines) damage the nervous system. Public health is maintained through personal hygiene, refuse disposal, sewage treatment, clean water supply, vector control, immunisation, and health education.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Epidemiological Terms and Common Confusions

NABTEB often tests the distinction between epidemic (sudden outbreak of a disease within a community or region), pandemic (worldwide spread, e.g. COVID-19 in 2020), and endemic (constantly present in a particular area, e.g. malaria in tropical West Africa). The incubation period is the time between pathogen entry and the first appearance of symptoms; it varies from a few days (cholera: 1–5 days) to several weeks (HIV: weeks to months) and determines quarantine durations.

Mechanism of HIV/AIDS — Why It Is a Virus, Not a Disease

A frequent NABTEB trap is treating HIV and AIDS as synonymous. HIV (Human Immunodeficiency Virus) is the retrovirus that infects CD4+ T-helper lymphocytes, destroying them progressively. AIDS (Acquired Immune Deficiency Syndrome) is the clinical condition that follows — characterised by severe immunodeficiency, opportunistic infections (e.g. Pneumocystis pneumonia, tuberculosis), and rare cancers such as Kaposi’s sarcoma. HIV is transmitted via sexual contact, contaminated needles, blood transfusion, and mother-to-child (placenta, breast milk), but not by casual contact.

Why Antibiotics Fail Against Viruses

Antibiotics (e.g. penicillin, streptomycin) disrupt bacterial cell-wall synthesis or protein synthesis. Viruses lack cell walls and use host cell machinery to replicate, so antibiotics are ineffective against colds, influenza, and measles. Antiviral drugs (e.g. acyclovir, antiretrovirals like zidovudine) target specific viral enzymes. This distinction is a common NABTEB objective question.

Worked Comparison: Types of Immunity

Type	Source of antibodies	Duration	Example
Natural active	Body produces them after infection	Long (years/lifetime)	Recovery from measles
Artificial active	Body produces them after vaccination	Long	Polio, BCG, measles vaccine
Natural passive	Mother to child (placenta/milk)	Short (weeks–months)	Maternal IgG in newborn
Artificial passive	Injection of antiserum	Short	Anti-rabies immunoglobulin

Common Exam Traps

• Confusing HIV (virus) with AIDS (disease syndrome).
• Saying antibiotics cure colds, influenza, or COVID-19.
• Mixing up Anopheles (malaria) with Aedes (yellow fever/dengue).
• Calling ringworm a worm — it is a fungal infection.
• Treating addiction and mental illness as moral failings rather than health disorders requiring treatment.

Practice Prompts

1. A NABTEB question may ask: Tabulate three differences between active and passive immunity, and give one example of each. Use the table above to draft a full answer.
2. Another likely objective: State the causative organism and one vector of (a) malaria, (b) sleeping sickness, (c) yellow fever, (d) cholera. Answers: (a) Plasmodium, Anopheles mosquito; (b) Trypanosoma brucei, tsetse fly; (c) yellow fever virus, Aedes mosquito; (d) Vibrio cholerae, housefly/faecal–oral route.

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