state that infectious diseases are caused by pathogens and are transmissible

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Infectious Diseases

Infectious Diseases

Learning Objective

State that infectious diseases are caused by pathogens and are transmissible.

Key Definitions

  • Infectious disease: An illness caused by the invasion and multiplication of pathogenic microorganisms within a host.

  • Pathogen: Any organism that can cause disease, including viruses, bacteria, fungi, protozoa and helminths.

  • Transmissible: Capable of being transferred from one individual (or species) to another.

Major Groups of Pathogens

Pathogen TypeTypical ExamplesKey Characteristics
VirusesInfluenza virus, HIV, SARS‑CoV‑2Obligate intracellular parasites; contain nucleic acid genome surrounded by protein coat.
BacteriaStreptococcus pyogenes, Mycobacterium tuberculosisProkaryotic cells; can be Gram‑positive or Gram‑negative; many produce toxins.
FungiCandida albicans, DermatophytesEukaryotic; grow as yeasts or moulds; cell wall contains chitin.
ProtozoaPlasmodium spp., Giardia lambliaSingle‑celled eukaryotes; often have complex life cycles involving vectors.
HelminthsAscaris lumbricoides, Schistosoma mansoniMulticellular parasites; include nematodes, trematodes, and cestodes.

Transmission Routes

  1. Direct contact – skin-to-skin or mucosal contact (e.g., herpes simplex).

  2. Indirect contact – via fomites such as clothing or surfaces.

  3. Droplet – respiratory droplets expelled when coughing or sneezing.

  4. Aerosol – fine particles that remain suspended (e.g., measles virus).

  5. Vector‑borne – carried by arthropods like mosquitoes (e.g., malaria).

  6. Food and water – ingestion of contaminated substances (e.g., cholera).

  7. Sexual – transmission through bodily fluids (e.g., HIV).

  8. Vertical – from mother to child during pregnancy, birth or breastfeeding.

Why Understanding Transmission Is Important

Control measures such as vaccination, hygiene, quarantine and vector control are based on interrupting the specific routes by which pathogens move between hosts.

Illustrative Diagram

Suggested diagram: Flowchart showing the relationship between pathogen type, transmission route, and preventive measures.

Summary Checklist

  • Infectious diseases are caused by pathogens.

  • Pathogens are transmissible between hosts.

  • Different pathogens use distinct transmission routes.

  • Prevention strategies target these routes.