Health is defined by the WHO as "a state of complete physical, mental, and social well-being and not merely the absence of disease." Disease is any abnormal condition that impairs body function. Diseases may be infectious (communicable) or non-infectious (non-communicable).
Common Human Diseases
- Bacterial diseases:
- Typhoid (Salmonella typhi): Widal test for diagnosis; transmitted via contaminated food and water; causes high fever, intestinal perforation in severe cases
- Pneumonia (Streptococcus pneumoniae / Haemophilus influenzae): Affects alveoli; transmitted by inhalation
- Common cold (Rhinoviruses): Mild upper respiratory infection
- Viral diseases:
- Malaria (Plasmodium — a protozoan): Vector — female Anopheles mosquito; species: P. vivax (benign tertian), P. falciparum (malignant/deadly), P. malariae (quartan), P. ovale
- Amoebiasis (Entamoeba histolytica): Protozoan; transmitted via contaminated food/water; causes amoebic dysentery
- Ascariasis (Ascaris lumbricoides): Nematode helminth; transmitted via soil/contaminated food; roundworm infection
Note: Malaria is caused by the protozoan Plasmodium, not a virus or bacterium. It is a parasitic disease.
Malaria — Life Cycle
- 1.Female Anopheles mosquito bites human → injects sporozoites into blood
- 2.Sporozoites travel to liver → multiply (pre-erythrocytic cycle) → hepatic schizonts → merozoites released
- 3.Merozoites infect RBCs → erythrocytic cycle → symptoms appear (chills, fever, shivering coincide with RBC lysis)
- 4.Some merozoites become gametocytes → taken up by another mosquito → sexual reproduction in mosquito
P. falciparum infects all age groups of RBCs and blocks blood vessels (cerebral malaria). P. vivax infects only young RBCs and has a dormant liver stage (hypnozoites). P. malariae infects older RBCs and causes quartan fever (every 72 hours).
Immunity
Innate immunity: Non-specific, present from birth. Includes: skin barrier, mucus, tears (lysozyme), phagocytes, natural killer cells, complement proteins, fever, inflammation.
- Acquired (Adaptive) immunity: Develops after exposure to a specific antigen. Two types:
- Humoral immunity: B-lymphocytes produce antibodies (immunoglobulins — IgG, IgA, IgM, IgD, IgE)
- Cell-mediated immunity (CMI): T-lymphocytes directly attack pathogens; important against intracellular pathogens and transplant rejection
Active immunity: Body produces own antibodies (natural — from infection; artificial — from vaccination). Long-lasting.
Passive immunity: Ready-made antibodies from outside (natural — maternal antibodies via placenta/colostrum; artificial — antiserum injection). Immediate but short-lasting.
Memory cells: When B and T cells encounter an antigen, some differentiate into long-lived memory cells. On subsequent exposure to the same antigen, the secondary immune response is faster and stronger (due to memory cells). This is the basis of vaccination.
A newborn receives passive immunity from mother — IgG antibodies cross the placenta and IgA is present in colostrum (first milk). This protects the infant while its own immune system matures.
Vaccines and Immunisation
Vaccines introduce antigens (weakened pathogen, killed pathogen, subunit, or toxoid) to stimulate immune response without causing disease, creating immunological memory. Examples: BCG (tuberculosis), DPT (diphtheria, pertussis, tetanus), OPV (polio), MMR (measles, mumps, rubella).
AIDS (Acquired ImmunoDeficiency Syndrome)
- Caused by HIV (Human Immunodeficiency Virus — retrovirus). HIV destroys CD4+ T-helper lymphocytes → immune deficiency → opportunistic infections (e.g., Pneumocystis carinii pneumonia, Toxoplasma, Candida).
- Transmission: sexual contact, contaminated blood/needles, mother-to-child (transplacental, breast milk)
- Diagnosis: ELISA (screening), Western blot (confirmation)
- Window period: 6-12 months (HIV present but antibodies not yet detectable)
- No cure; managed with Antiretroviral Therapy (ART)
HIV is a retrovirus — its genome is RNA. Inside the host cell, reverse transcriptase converts viral RNA → DNA → integrates into host genome (provirus). This makes HIV difficult to eliminate.
Cancer
- Cancer results from uncontrolled cell division due to mutations in proto-oncogenes (→ oncogenes) or tumour suppressor genes. Types:
- Carcinoma: Epithelial tissue cancer (most common)
- Sarcoma: Connective tissue cancer
- Lymphoma/Leukaemia: Blood and lymphoid tissue cancer
Metastasis: Cancer cells spreading to other organs via blood/lymph.
Causes (carcinogens): Ionising radiation, tobacco smoke, UV radiation, oncogenic viruses, chemicals.
Alpha-1-fetoprotein (AFP) and PSA (Prostate Specific Antigen) are tumour markers used in blood tests to detect liver cancer and prostate cancer respectively. Early detection improves survival.
Tobacco smoke contains benzpyrene and other carcinogens. Smoking causes lung cancer, throat cancer, and contributes to other cancers. It is estimated to cause 80% of lung cancer cases.
Drugs and Alcohol Abuse
Common drugs of abuse: opioids (heroin, morphine — act on opioid receptors), cannabinoids (marijuana, hashish — act on cannabinoid receptors in brain), cocaine (stimulant, addictive), amphetamines, barbiturates, LSD.
Heroin (diacetylmorphine) is synthesised by acetylation of morphine. It crosses blood-brain barrier rapidly, causing euphoria. Repeated use leads to tolerance (need higher doses) and dependence (withdrawal symptoms on cessation). Sharing needles spreads HIV and hepatitis B.
Withdrawal symptoms: Nausea, cramps, anxiety, sweating — occur when the drug is discontinued in addicted individuals.
Common mistakes
- Malaria is caused by Plasmodium (a protozoan), NOT a bacterium or virus.
- Passive immunity is short-lived; active immunity is long-lived.
- In AIDS, HIV attacks CD4+ T-helper cells, NOT B-cells or red blood cells.
- Cancer cells undergo metastasis — they spread to distant organs, making cancer hard to treat.
- ELISA is used for HIV screening; Western blot is the confirmatory test.
Summary
Human diseases range from bacterial and parasitic infections to viral diseases like AIDS and cancer. Immunity — innate and adaptive — protects the body. Vaccination creates artificial active immunity. HIV destroys CD4+ T cells, leading to AIDS and opportunistic infections. Cancer arises from uncontrolled cell division. Drug and alcohol abuse affects health through addiction and organ damage.