Background
Infectious diseases are disorders that are caused by organisms, usually microscopic in size, such as bacteria, viruses, fungi, or parasites that are passed, directly or indirectly, from one person to another. Humans can also become infected following exposure to an infected animal that harbors a pathogenic organism that is capable of infecting humans.
Infectious diseases are a leading cause of death worldwide, particularly in low-income countries.
Infectious diseases can be caused by several different classes of pathogenic organisms (commonly called germs). These are viruses, bacteria, protozoa, and fungi. Almost all of these organisms are microscopic in size and are often referred to as microbes or microorganisms.
Examples of diseases caused by viruses are COVID-19, influenza, HIV/AIDS, Ebola, diarrheal diseases, hepatitis, and West Nile. Diseases caused by bacteria include anthrax, tuberculosis, salmonella, and respiratory and diarrheal diseases.
There are several different routes by which a person can become infected with an infectious agent. For some agents, humans must come in direct contact with a source of infection, such as contaminated food, water, fecal material, body fluids, or animal products. With other agents, infection can be spread through the air.
A chain of infection is a model to help characterize an infectious agent to better understand how it spreads and develop appropriate prevention strategies. A chain of infection includes six components.
- Infectious Agent: Disease producing, also called pathogens.
- Virus, parasite, fungus, bacterium
- Reservoir/Source: Environment/habitat where a pathogen can live and multiply.
- Environmental surfaces/equipment, body fluids (blood, saliva), urine/fecal material, food/water, soil, skin, respiratory tract.
- Portal of Exit: How the pathogen exits or leaves reservoir.
- Skin to skin, skin to surface, blood, mucous membranes, oral cavity, feces, cough.
- Modes of Transport: How a pathogen moves from reservoir to susceptible host.
- Direct Transmission: Airborne, droplet, contact, bite, needle stick or other sharps injury.
- Indirect Transmission: Fomites – contaminated equipment or medication, rodents/insects, food, water.
- Portal of Entry: Opening where the pathogen may enter.
- Body openings, incisions, wounds.
- Susceptible Host: The individual(s) at risk.
- Factors affecting susceptibility include age, health, immune system, nutrition.
Additional Background Resources:
Centers for Disease Control and Prevention – Infection Control: https://www.cdc.gov/infectioncontrol/spread/index.html
Facilitator Note
Two versions of this lesson are provided. Version A includes zoonotic diseases in the Chain of Infection activity and is intended for learners who have prior knowledge of One Health. Version B is intended for learners who have not yet had experience with the concept of One Health. The Chain of Infection activity includes only non-zoonotic diseases, and the concept of zoonoses and the One Health approach is included in a follow-up activity that includes reading and discussion of an article reviewing the role of rodents in global-health events.