Types of Disease & Pathogens

Types of Disease

  • Disease = any condition that disrupts normal structure/function of an organism.
  • Two broad classes:
    • Non-infectious: not communicable; arise from genetic, behavioural, physiological or environmental factors.
    • Infectious: communicable; caused by pathogens; transmitted host→host (directly, via vectors, or opportunistically).

Non-infectious Disease Factors

  • Genetic: DNA mutations (e.g. cystic fibrosis, haemophilia).
  • Environmental: air pollution, radiation.
  • Behavioural: diet, exercise, drug use.
  • Physiological: high BP, obesity, hyperglycaemia\text{high BP},\ \text{obesity},\ \text{hyperglycaemia}.
  • Sample diseases: rickets (vitamin D deficiency), coal worker’s pneumoconiosis.

Infectious Disease Basics

  • Pathogen = agent that causes disease; infected organism = host.
  • Vectors carry pathogens between hosts (e.g. mosquitoes).
  • Opportunistic pathogens cause illness when host immunity is weakened.

Viruses

  • Non-cellular; protein coat + nucleic acid (DNA or RNA).
  • Replication: inject genome → hijack host machinery → assemble virions → host cell lysis.
  • Highly host-/cell-specific (e.g. adenovirus in respiratory epithelium; bacteriophages in bacteria).
  • Always pathogenic; rely on host for reproduction.

Prions

  • Non-cellular, misfolded proteins; no nucleic acid.
  • Convert normal proteins → prion form → exponential accumulation.
  • Transmission: ingestion of infected tissue; highly resistant to sterilisation.
  • Outcome: neurodegeneration (e.g. Creutzfeldt-Jakob disease); invariably fatal, no treatment.

Bacteria

  • Prokaryotic; majority non-pathogenic, form human microbiome.
  • Key structures: cell wall, capsule, flagellum; can form heat-resistant endospores.
  • Reproduce by binary fission ⇒ rapid population growth.
  • Shapes: cocci (spherical), bacilli (rod), spirilla (spiral), vibrios (comma-shaped).

Fungi

  • Eukaryotic; cell walls of chitin; reproduce via spores.
  • Secrete enzymes outside body → absorb digested nutrients.
  • Animal diseases mainly external (ringworm, tinea); also major plant pathogens (e.g. Botrytis cinerea).

Protists

  • Unicellular eukaryotes; < 0.04%0.04\% of species infect humans.
  • Examples: Giardia, Cryptosporidium (GI illness); Plasmodium spp. cause malaria (via Anopheles mosquitoes, infect RBCs).

Parasites

  • Live on/within host, causing harm.
  • Endoparasites: internal, multicellular (tapeworms, roundworms, flukes); absorb nutrients, release eggs in faeces.
  • Ectoparasites: external (ticks, fleas, lice); mild symptoms but may transmit other pathogens (e.g. Lyme disease).