Infectious diseases are caused by organisms like bacteria, viruses, fungi, or parasites. To stop and manage the disease-causing agent, we need to understand how they spread. That is why understanding the chain of infection is important.
Let’s look at the chain of infection in more detail:
Infectious agent
Infectious agents, also known as pathogens, are infection-causing microorganisms, including bacteria, viruses, fungi, and parasites.
Reservoir
The reservoir of an infectious agent is where the agent typically lives, grows, and multiplies. This can be found in humans, animals, or the environment. The reservoir may or may not be the actual source of the agent that infects a host. For instance, the reservoir of Clostridium botulinum is soil. Still, most botulism infections come from improperly canned food with C. botulinum spores.
Portal of exit
The portal of exit is the path a pathogen uses to leave its host. It usually corresponds to where the pathogen is located in the host’s body. For example, influenza and TB exit through the respiratory tract, while cholera and schistosomes exit through feces and urine. Some bloodborne pathogens exit via the placenta (rubella, syphilis, toxoplasmosis). In contrast, others use cuts (hepatitis B) or blood-sucking arthropods (malaria).
Mode of transmission
Transmission happens when an infectious agent moves from its original source or host through an exit point, travels through a specific mode of transfer, and enters a susceptible host through an entry point, leading to an infection.
The primary transmission routes include direct contact, indirect contact, droplet transmission, airborne transmission, vector-borne transmission, and fecal-oral path.
- Direct contact – The disease can spread when an infected person comes into physical contact with another person. Examples include touching or shaking hands with an infected person. STIs including HIV, syphilis, and gonorrhea are a few examples.
- Indirect contact – Pathogens can also spread through contaminated surfaces or objects. For instance, touching a door handle previously handled by someone with a common cold could pass on the infection to the uninfected person.
- Droplet transmission – COVID-19 can spread through droplets produced when an infected person coughs or sneezes.
- Airborne transmission – Infectious agents can remain suspended in the air over long distances and periods. Tuberculosis is one disease known to be transmitted this way. Measles is one of the examples.
- Fecal-oral route – The fecal-oral route involves pathogens passing from feces to mouth – usually due to poor sanitation practices. Norovirus would be a classic example.
- Vector-borne transmission – Vector-borne diseases are transmitted via insects such as mosquitoes. Vector-borne infections like malaria, dengue, and Lyme disease pose significant health risks worldwide.
Portal of entry
The portal of entry is how a pathogen enters a susceptible host, providing access to tissues where it can multiply, or toxins can act. Many infectious agents use the same entry point to exit the source host and enter a new host. For example, the influenza virus exits and enters the respiratory tract. Others causing gastroenteritis follow a “fecal-oral” route, leaving through feces, contaminating hands, and entering a new host through the mouth via contaminated food, water, or utensils. Additional entry portals include the skin (Hookworm), mucous membranes (Syphilis), and blood (Hepatitis B, HIV).
Susceptible Host
A susceptible host is a person or animal who is not immune or resistant to the infectious agent and can contract the disease if exposed. The following factors make a person more susceptible to infections:
- Age – Older adults and children are more vulnerable to flu and pneumonia.
- Health conditions – diabetes, heart disease, cancer treatments, and HIV/AIDS.
- Unhealthy lifestyle choices – smoking, excessive alcohol consumption, and poor diet.
- The environment – living in crowded or unsanitary conditions.