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The etiology of disease is complex and involves a combination of biological, behavioral, social, and environmental factors. Here are three widely used models of disease causation:
Epidemiologic Triad or Triangle: One of the simplest models of disease causation. The triad consists of the causative agent(s), the host, and the environment. The epidemiological triad is a simple but elegant model of depicting the balance between the agent, the host, and the environment. This balance is important, even for the simplest of diseases. Understanding the three major parts of the “disease equation" also enables us to think about where interventions should be directed (e.g., toward the agent, host, or environment).
Chain of Infection: This model recognizes that a series of events have to happen to cause infections in a person. We can think of each part of the infection process as a separate “link” in the chain. The links include the infectious organism, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host. If we can break a link at any part of the chain, we can stop infection.
Causal Pie: This model is often used to describe components of diseases that are not communicable, such as diabetes or hypertension. There can be many pieces of the pie, but all pieces are labeled with one of these three labels: component cause, sufficient cause, and necessary cause. Each piece of the pie is called a component cause. The entire pie is called a sufficient cause. The piece that must be present for the disease to be present is called the necessary cause.
Mode of Transmission
Direct Contact: infectious particles enter through skin. This can happen through touching, kissing, sexual contact, etc. Examples are herpes and other STDs.
Droplet: infectious particles greater than 5 microns that enter into mucous membranes. Examples include strep pharyngitis, diphtheria, and pertussis.
Airborne: infectious particles less than 5 microns in size that can be carried by air. Examples include measles, influenza, and now Covid-19.
Vehicleborne: infectious particles are carried by food, water, blood products, and/or fomites. Examples include botulism and Hepatitis A.
Vectorborne: infectious particles are carried by flies, ticks, fleas, mosquitoes. Examples include Dengue fever, West Nile Virus, Lyme disease, and malaria.