Fundamentals - Principles, part 11: The Chain and Stages of Infection

July 19, 2021 Updated: August 30, 2021 7 min read

In this article, we'll cover the chain of infection and the stages of infection. We'll use COVID-19 as an example to help you understand some of these steps!

The Fundamentals of Nursing video series follows along with our Fundamentals of Nursing flashcards, which are intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI, and NCLEX.

The chain of infection

The chain of infection describes how a pathogen gets from where it is to where it's going. It starts with the infectious agent, which lives in a reservoir, finds a portal of exit, and uses a mode of transmission to find a portal of entry into a susceptible host.

Infectious agent

First in the chain of infection is the infectious agent, which is an organism capable of causing infection or disease. Examples of infectious agents include bacteria, viruses, fungi, and parasites.

For example, COVID-19 is an infectious agent.

Reservoir

The reservoir is where the infectious agent lives and multiplies. A reservoir could be a body of water, human, or animal.

In the case of COVID-19, a person could be a reservoir for the virus.

Portal of exit

The portal of exit is the way that the infectious agent leaves its reservoir. If a person is the reservoir for the pathogen, the portal of exit could be their nose and mouth when sneezing, their blood (a bloodborne pathogen), or the infectious agent could exit fecally.

For example, COVID-19's portal of exit is the nose and mouth, which is why masks are worn.

Mode of transmission

The mode of transmission explains how the infectious agent gets from the reservoir to the new host. The portal of exit is just how the pathogen exited, but the mode of transmission explains how it travels from there. This can include direct contact, respiratory droplets, airborne, vehicles like water or food, or vectors like mosquitoes, ticks, or fleas.

In the case of COVID-19, its mode of transmission is respiratory droplets. This is why social distancing and limited gatherings are essential to stop the spread; fewer people around each other, fewer respiratory droplets flowing through the air. If you have COVID and you and your respiratory droplets are alone in your home, they can't reach the next item on the list—a portal of entry to a susceptible host.

Portal of entry

The portal of entry is how the infectious agent enters into the new host. This could be through the nose and mouth, through the eyes, or through the bloodstream (e.g., into an open wound). The portal of entry may be the same as the portal of exit.

For example, COVID-19's respiratory droplet portal of entry is the nose and mouth, which is again why masks are worn to protect against the virus.

Susceptible host

A susceptible host is the recipient of the infection. The host must be susceptible; and not all hosts are susceptible. Hosts might not be susceptible because they have had the disease previously and developed antibodies against it. Some hosts have stronger immune systems than others, e.g., someone who is immunocompromised is a more susceptible host.

In the case of COVID-19, getting vaccinated makes you a less susceptible host.

The stages of infection

The stages of infection describe what happens once a susceptible host has acquired a pathogen. The stages are: incubation, prodromal, illness, and convalescence.

Incubation stage

The incubation period of infection is the time from when the pathogen first gets in the body, until it starts to make its appearance known. During this time, the pathogen is multiplying inside your body. One does not know they are sick at this time.

Diseases or infections with a long incubation period are concerning because a person might be contagious long before they know they are sick, and they may not know to take precautions against spreading the illness.

Prodromal stage

The prodromal period of infection is when general, non-specific signs and symptoms of illness appear. These symptoms are vague, not specific to any one disease.

If you have ever woken up in the morning and thought, "I don't really feel very well. I can't tell you what's wrong. I'm just tired. I don't feel good. I feel kind of fatigued and weak," that was the prodromal stage at work.

Illness stage

During the illness phase, infection-specific signs and symptoms appear.

For example, a patient may have a high fever, a cough, and body aches if they are in the illness phase of influenza. If a patient has gastroenteritis, they might have an upset stomach, cramps, nausea, vomiting, or diarrhea.

If you'd like to learn the key facts about influenza, gastroenteritis, and more infections, diseases and disorders, check out our best-selling Medical-Surgical Nursing Flashcards.

Convalescence stage

Last in the stages of infection is the convalescence stage, which is the recovery from infection. To convalesce means to get better. During the convalescence stage, symptoms start to disappear until a person is fully healed.

If an illness is something like the common cold, the convalescence stage might be just a few days. If it's a serious or long-lasting infection, the convalescence stage could take a very long time.


Full Transcript

Hi. I'm Meris, and in this video, we're going to be talking about the chain and stages of Infection. I will be following along using our Fundamentals of Nursing flashcards. If you would like to get a set for yourself, they are available on our website at levelupRN.com. If you already have a set and you're following along with me, I am starting on card number 32. So let's get started.

So first up, we are talking about the actual chain of infection, which describes how the pathogen gets from where it is to where it is going. So first, we have the infectious agent.

So what is it? Is it a virus? Is it a bacteria? That kind of thing describes what the pathogen itself is. So E coli, for instance, very different than the flu, influenza A or B.

Then the reservoir, so that's where the pathogen lives and multiplies. So maybe that's going to be in a body of water, for instance. Or maybe it's going to be inside of another living creature. So that's going to describe the reservoir.

Now the portal of exit is the way that the pathogen gets from wherever its reservoir is, to something else. So for instance, if I am the reservoir for the pathogen, the portal of exit could be my nose and mouth when I sneeze. Right? It could be in my blood, right, bloodborne pathogens. It could be fecal. It could come out in somebody's poop. So all of those are going to describe different portals of exit.

And then we're going to get to the mode of transmission. So the mode of transmission is actually going to explain how the agent goes from that reservoir to the new host.

So yeah, I had the portal of exit, but is it direct contact? Do I have to actually make contact with somebody? Is it droplet and it's spread through respiratory droplets? Is it airborne and spread through the tiny airborne particles that don't drop to the floor? All of those things describe the mode of transmission, how it gets from one place to the other.

Then we have the portal of entry. How does it get into the new host, right? Is it going to be-- if it's a droplet or airborne disease, it might be through my nose and mouth. That could also be the portal of entry. It could be by touching my eyes, right, rubbing my eyes. That's why we try not to touch our faces. So that's going to describe portal of entry.

And then lastly, the host. The host must be susceptible. So not all hosts are susceptible. Some of them have either had the disease prior and they have antibodies against it, or maybe they have really strong immune defenses and are not susceptible compared to somebody with a weakened immune system or maybe somebody who is older in age and has just a naturally weaker immune system. So that is the chain of infection.

And then I want to show you here on card number 33, we have a really nice drawing of the chain of infection and what all the components are. In my opinion, I think this is one of the best illustrations of the chain of infection, and it actually kind of makes more sense than what you find in the textbook. So definitely check that out in our deck.

Okay. So moving right along, we have stages of infection. This is on card number 34. So this describes, once the susceptible host has gotten the pathogen, what happens.

So first up, we have the incubation period. So the incubation period is the time from when the pathogen first gets in the body until it starts to make its appearance known. So this is going to be when the pathogen's multiplying inside your body. You have no idea that you're even sick. Diseases or infections that have a long incubation period are a big deal because I might be contagious way before I ever know that I'm sick. So that's why it's important to understand what the incubation period is.

Now the prodromal period, the prodrome is when I start to have initial symptoms, but these symptoms are vague. They are not specific. This is when you wake up in the morning and you go, "I don't really feel very well. I can't tell you what's wrong. I'm just tired. I don't feel good. I feel kind of fatigued and weak." That's the prodromal period.

Now, when I start to have actual specific symptoms related to the pathogen, that's going to be the actual illness phase. So when I have a high fever, a cough, body aches, all of those things, that's when I'm in the illness phase of influenza, for instance. It's going to be specific to the disease.

So now, if I have gastroenteritis, that's going to be upset stomach, cramps, nausea, vomiting, diarrhea. All of those would be in the illness phase. So it's specific to the pathogen.

And then lastly is the convalescence stage. So to convalesce means to get better. So that's going to be once those symptoms start to disappear until the person is fully healed. Now, if it's something like the common cold, it's just going to be a couple of days, right, but now if we're talking about something that's very serious or long lasting, that convalescent stage could take a very long time.

So that is covering the chain of infection and the stage of infection. I hope that was a helpful review for you. If it was, please like this video. Drop a comment if you've got anything to share with us, a really great way to remember something or anything like that. We would love to hear from you.

In the next video, I'm going to be covering types of immunity and illness, so be sure that you subscribe to the channel because you want to be the first to know when that gets posted. Thanks so much, and happy studying.


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