2.1 Bacterial Diseases

• Typhoid — pathogen: Salmonella typhi; transmission: contaminated food/water; target: small intestine; key features: sustained fever, intestinal perforation; diagnosis: Widal test.
• Pneumonia — pathogens: Streptococcus pneumoniae, Haemophilus influenzae; transmission: droplet inhalation; key feature: fluid-filled alveoli impairing gas exchange; diagnosis: chest X-ray, sputum culture.

2.2 Viral Diseases

• Common cold — pathogen: Rhinovirus; target: upper respiratory tract; self-limiting illness, no specific diagnostic test.
• AIDS/HIV — pathogen: HIV (retrovirus); key enzyme: reverse transcriptase (RNA → DNA integration into host genome); target cell: CD4+ T-helper lymphocytes; transmission: sexual contact, contaminated blood/needles, mother-to-child; diagnosis: ELISA (screening), Western blot (confirmation); consequences: progressive immunodeficiency, opportunistic infections.

2.3 Protozoan Diseases

• Malaria — pathogens: Plasmodium vivax (benign tertian, 48-hr fever cycle), P. falciparum (malignant tertian, most lethal); vector: female Anopheles mosquito; cycle: sporozoites → liver schizogony → merozoites → RBC schizogony → haemozoin release $\frac{fever}{chills}$ → gametocytes → mosquito.
• Amoebiasis — pathogen: Entamoeba histolytica; target: large intestine; transmission: contaminated food/water; symptoms: abdominal pain, bloody diarrhoea; diagnosis: stool microscopy.

2.4 Helminth Diseases

• Ascariasis — pathogen: Ascaris lumbricoides; target: small intestine; symptoms: intestinal blockage, muscular pain; diagnosis: stool examination.
• Filariasis (Elephantiasis) — pathogens: Wuchereria bancrofti, W. malayi; vector: Culex mosquito (NOT Anopheles); target: lymphatic vessels; symptoms: chronic lymphoedema, swelling of lower limbs.

2.5 Immunity

• Innate immunity — non-specific; four barrier types: physical (skin), physiological (lysozyme, acid), cellular (macrophages, NK cells, neutrophils), cytokine (interferons); immediate response, no memory.
• Adaptive immunity:
  • Humoral: B-cells → plasma cells → antibodies (IgA, IgD, IgE, IgG, IgM); antigen-specific; memory B-cells formed.
  • Cell-mediated: T-cells (CD4+ helper, CD8+ cytotoxic, suppressor); memory T-cells formed.
• Active immunity: self-produced antibodies; slow onset; long-lasting; memory cells.
• Passive immunity: received preformed antibodies; immediate onset; short-lived; no memory cells.
• Allergy: IgE + mast cells + histamine; exaggerated response to harmless antigens.
• Autoimmunity: self-antigen attack; examples: rheumatoid arthritis, SLE.

2.6 Cancer

• Carcinogens: physical (UV, radiation), chemical (tobacco), biological (oncogenic viruses).
• Oncogenes: activated proto-oncogenes promoting uncontrolled division.
• Tumour suppressors: p53, Rb — when lost, cancer can develop.
• Benign vs malignant: benign = localised, encapsulated; malignant = invasive, metastatic.
• Detection: biopsy, CT, MRI, PET, molecular markers.
• Treatment: surgery, radiation, chemotherapy, immunotherapy.

2.7 Substance Abuse

• Opioids: Heroin (diacetylmorphine) from Papaver somniferum; CNS depressant; highly addictive.
• Cannabinoids: Marijuana/hashish from Cannabis sativa; cardiovascular effects.
• Cocaine: From Erythroxylum coca; blocks dopamine reuptake; CNS stimulant.
• Nicotine: Tobacco; stimulant + carcinogen; causes lung cancer, emphysema.
• Alcohol: CNS depressant; liver cirrhosis with chronic use.