Communicable Disease Transmission: Seven Routes of Spread

How Infectious Agents Move Between Hosts

Communicable diseases are caused by pathogens (bacteria, viruses, fungi, parasites, prions) transmitted via specific routes:

1. Direct Contact

Physical touch with infected person/animal: skin-to-skin (HSV, HPV, scabies), sexual (HIV, syphilis, gonorrhea), droplet spray (cough/sneeze → mucosa within 1–2 m; influenza, colds, COVID-19).

2. Indirect Contact (Fomites)

Contaminated inanimate objects: doorknobs, phones, towels, medical equipment. Pathogens survive minutes to months (norovirus weeks, C. diff spores months). Hand hygiene critical.

3. Airborne (Aerosol) Transmission

Droplet nuclei (<5 µm) remain suspended, travel distances, inhaled. Requires airborne precautions (N95, negative pressure). Examples: tuberculosis, measles, varicella, SARS-CoV-2 (in aerosol-generating contexts).

4. Fecal-Oral

Ingestion of contaminated food/water. Vehicles: unwashed produce, undercooked shellfish, contaminated water supply, poor hand hygiene. Diseases: cholera, typhoid, hepatitis A, rotavirus, norovirus, pinworm.

5. Vector-Borne

Arthropod transmits pathogen during blood meal.

• Mosquitoes: malaria, dengue, Zika, yellow fever, West Nile, chikungunya.
• Ticks: Lyme, RMSF, anaplasmosis, babesiosis.
• Fleas: plague, murine typhus.
• Triatomine bugs: Chagas disease.

6. Blood-Borne

Exposure to infected blood: needle sharing (HIV, HCV, HBV), transfusion (screened), needlestick, perinatal. Also sexual (HIV, HBV, HCV).

7. Vertical (Mother-to-Child)

• Transplacental (congenital): rubella, CMV, toxoplasmosis, syphilis, Zika, HIV.
• Peripartum: birth canal exposure (HSV, HIV, HBV, GBS).
• Breastfeeding: HIV (in resource-limited settings without ART), CMV, HTLV-1.

Breaking the Chain

Interventions target specific routes: vaccination, sanitation, vector control, safe injection, PPE, isolation, antimicrobial stewardship.