Understanding how diseases spread is first step in stopping them. In public health, this process is known as Chain of Infection. Think of it as a six-link circuit; if you break just one link, entire cycle of disease transmission collapses.
Six Links of Chain
For an infection to occur, a pathogen must travel through each of these steps in a specific order:
1. Infectious Agent (Pathogen)
This is germ that causes disease. It could be a virus, bacterium, fungus, or parasite. Whether or not an agent causes an infection depends on its virulence (strength), dose, and its ability to enter body.
2. Reservoir
This is home where pathogen lives, grows, and multiplies.
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Humans: People showing symptoms or carriers who have no symptoms.
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Animals: Known as zoonotic reservoirs (e.g., rabies in bats).
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Environment: Contaminated water, soil, or food.
3. Portal of Exit
To spread, the germ needs a way out of reservoir. Common exit points include:
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Respiratory tract (coughing, sneezing).
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Gastrointestinal tract (feces, saliva).
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Blood or body fluids.
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Skin lesions.
4. Mode of Transmission
This is how germ travels from exit point to a new person. This is often easiest link to break.
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Direct Contact: Touching, kissing, or sexual contact.
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Indirect Contact: Touching a contaminated surface (fomite) like a doorknob.
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Droplet: Large particles that travel short distances (flu, COVID-19).
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Airborne: Tiny particles that present for long time in air (measles, TB).
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Vector-borne: Carried by insects like mosquitoes or ticks.
5. Portal of Entry
Germ needs a doorway into next person. Interestingly, these are often same as portals of exit:
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Inhalation (breathing it in).
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Ingestion (eating contaminated food).
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Absorption through mucous membranes (eyes/nose).
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Breaks in skin (cuts or needle sticks).
6. Susceptible Host
Final link is person who receives the pathogen. Not everyone who encounters a germ gets sick. Susceptibility depends on:
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Age (very young and very old are more at risk).
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Immune status (vaccinations or previous exposure).
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Underlying health conditions or stress.
How to Break Chain
Public health interventions are designed to prevent these links from continuing. Here is how we do it:
| Link | How to Break It |
| Infectious Agent | Rapid diagnosis, antibiotics, or sterilization. |
| Reservoir | Environmental cleaning, pest control, and water treatment. |
| Portal of Exit | Wearing masks, using tissues, and proper hand hygiene. |
| Transmission | Handwashing, social distancing, and disinfecting surfaces. |
| Portal of Entry | Wound care, PPE (gloves/masks), and mosquito nets. |
| Susceptible Host | Vaccination, healthy diet, and preventative medicine. |
Pro Tip: Hand hygiene is widely considered single most effective way to break transmission link in almost any setting.