🦠📚 Communicable Disease Transmission: Sources & Reservoirs

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🔍 Definition

✅ A communicable disease is an illness caused by an infectious agent or its toxins that can be transmitted from:

• Person to person
• Animal to person
• Environment to person

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🧪💥 SOURCE vs. RESERVOIR – Important Concept!

🔹 Term	🧾 Definition
🏥 Source	The immediate origin from where the infection is acquired
🧫 Reservoir	The natural habitat where the organism lives, multiplies, and survives

🔁 Sometimes, source and reservoir are same (e.g., TB – human) or different (e.g., Tetanus: soil is reservoir, wound is source)

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📍 Examples of Reservoirs

🔬 Reservoir Type	🔄 Examples
🧍‍♂️ Human	TB, measles, typhoid, hepatitis, HIV/AIDS
🐕 Animal	Rabies (dog), plague (rat), brucellosis (cattle), leptospirosis (rodents)
🌱 Environment	Tetanus (soil), cholera (water), histoplasmosis (soil with bird droppings)

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🔍 Human Reservoir – Types

🧍 Type of Carrier	🧾 Explanation	💉 Example
👨‍🔬 Active Carrier	Person with symptoms	Measles, flu
🚶‍♂️ Healthy Carrier	No symptoms but can spread disease	Typhoid (Typhoid Mary)
🧑‍🦽 Incubatory Carrier	In incubation period, before symptoms	HIV, Hepatitis B
♻️ Convalescent Carrier	Recovering but still infectious	Cholera, diphtheria

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🧬 How Source & Reservoir Relate to Disease Control?

🔧 Strategy	🔑 Target
🚫 Isolation	Source – prevent spread from infected person
🧪 Treatment	Eliminate infection from reservoir
💉 Immunization	Protect host from reservoir or source exposure
🧼 Sanitation	Reduce environmental reservoirs (e.g., feces, water)

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🧠 💡 Golden Points for Competitive Exams

• ✅ Reservoir is the permanent dwelling place, source is the immediate transmitter
• ✅ All sources are not reservoirs
• ✅ Carriers can be symptomatic or asymptomatic
• ✅ Zoonoses are diseases where animal is the reservoir
• ✅ Disinfection of water interrupts environmental reservoir transmission

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📘 Example Question

Q. In case of tetanus, the source of infection is ____ and the reservoir is ____.
A. Water, Human
B. Wound, Soil
✅ C. Wound, Soil
D. Soil, Wound

Rationale: The wound is where the infection enters (source), and soil is where the bacteria live and multiply (reservoir).

🌟🦠 Communicable Disease Transmission: Modes of Transmission 🌟🦠

🔍 Definition: The mode of transmission refers to how an infectious agent moves from its source or reservoir to a susceptible host, enabling disease spread.

🔖 Modes of Transmission:

🛑 1. Direct Transmission:

• Direct Contact:
  • Transmission through physical touch (skin-to-skin, kissing, sexual contact).
  • Examples: STDs, scabies, herpes simplex.
• Droplet Spread:
  • Transmission through respiratory droplets expelled during coughing, sneezing, or talking (distance ≤ 1 meter).
  • Examples: Influenza, COVID-19, pertussis.

🔄 2. Indirect Transmission:

• Airborne Transmission:
  • Infectious agents travel in aerosols or dust particles suspended in the air (distance > 1 meter).
  • Examples: Measles, tuberculosis, varicella.
• Vehicle-Borne Transmission:
  • Transmission via contaminated non-living objects (food, water, fomites, blood).
  • Examples: Cholera (water), Salmonella (food), HIV (contaminated blood/products).
• Vector-Borne Transmission:
  • Transmission through vectors (living organisms like insects or animals).
    • Mechanical Vector: Pathogen carried externally by vector.
      • Example: Typhoid, cholera (houseflies).
    • Biological Vector: Pathogen multiplies or develops within the vector.
      • Example: Malaria, dengue (mosquitoes).

📌 Special Transmission Modes:

• Vertical (Transplacental): Mother-to-child transmission during pregnancy.
  • Examples: HIV, rubella.
• Nosocomial (Hospital-acquired): Through medical procedures or hospital environment.
  • Examples: MRSA, catheter-associated infections.
• Perinatal Transmission: Occurs during childbirth.
  • Example: Hepatitis B.

🧠 Key Points for Exams:

• Droplet transmission (<1 meter) vs. airborne transmission (>1 meter).
• Vehicle transmission involves inanimate objects.
• Biological vectors support pathogen lifecycle; mechanical vectors do not.
• Proper sanitation, vaccination, and isolation effectively interrupt transmission chains.

📝 MCQ for Review: Q: Tuberculosis primarily spreads through which mode of transmission?

• A) Vehicle-borne
• B) Direct contact
• ✅ C) Airborne
• D) Vector-borne

Rationale: Tuberculosis bacteria remain suspended in air particles, enabling transmission over distances greater than one meter, characteristic of airborne transmission.