Infection Transmission & Susceptibility

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Avoiding infection in medical settings is critical, especially in people who are already sick or injured. In this lesson we'll learn about how a person becomes infected and risk factors that make them susceptible to infection.

Pathogens and Infection

If you've ever watched a medical drama on television, you've seen a doctor 'scrub in' before surgery, but do you know why they do this? Your entire body is covered in microorganisms (don't cringe, many of them help you), but usually we can live harmoniously with these tiny critters without a problem. However, when certain microorganisms find their way into the body, they can become dangerous. When a microorganism causes an infection, it's known as a pathogen. Pathogens can be bacteria, viruses, prions, protozoans, fungi, or helminths.

Chain of Infection

How does something or someone become infected by a pathogen? There are a few steps that comprise the chain of infection. First, an infectious agent, or pathogen, must be present, and it must have an appropriate environment to live in. Most microorganisms require certain temperatures, pH levels, and moisture levels to thrive. Then it has to have a means to exit its current host (the organism it's living in), find a way to another host (a mode of transmission), and find a way into that new host's body. Finally, that new host has to offer a suitable environment for the pathogen to thrive and reproduce, causing an infection.

The body has natural defense mechanisms. The skin acts as an initial barrier; our mucous membranes can filter microorganisms out before they get into the body; and our immune system attacks foreign bodies to prevent infections from spreading. However, sometimes these defense mechanisms are compromised or, for whatever reason, they fail.

Transmission Routes

Now that we know what pathogens are and about the chain of infection, how exactly does a pathogen move from one host to another? In other words, how is someone introduced to a pathogen in the first place, and how does that pathogen get into the body? Transmission can occur via air, direct contact, or through a host of intermediate vehicles. Let's look at each of these a bit closer.

  1. Airborne transmission: Respiratory illnesses like the flu are often passed between people through the air. You can become infected by inhaling contaminated particles or droplets.
  2. Direct contact transmission: A person can become infected when they come into direct contact with a potential pathogen. This occurs during sexual contact, kissing, or even by touching contaminated clothing, utensils, dishes, door knobs, etc. Picking up pathogens on your hands and inadvertently introducing them to your body (for example by touching your mouth or nose) is one of the primary ways people get sick. This is why you're always advised to wash your hands frequently, especially during cold season! Microorganisms can live for brief periods on objects before needing a host to survive.
  3. Intermediate vehicle transmission: In addition to breathing pathogens in or directly touching them, they can be introduced indirectly through a variety of methods.
  • Blood-borne transmission: This can happen if a person becomes infected by receiving contaminated blood during a blood transfusion (though donated blood usually goes through thorough testing before it's cleared to be administered to a patient). Infection can also occur through contaminated medical or dental equipment, through a contaminated needle, or even through direct contact with contaminated blood on an open wound or mucous membrane.
  • Food-borne transmission: Someone can become infected by eating contaminated food products. This is a common transmission method for the bacteria Escherichia coli (commonly called E. coli).
  • Vector-based transmission: A person can become infected by being bitten by critters like bats, ticks, fleas, flies, or mosquitoes. These organisms often act as hosts for pathogens that harm humans.

Susceptibility to Infection

There are many different ways someone can be introduced to harmful microorganisms, but remember the host must be susceptible to infection. What are some risk factors that increase the likelihood an introduced microorganism will cause an infection? These include:

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