Nurse Aide Infection Control
Module 2: Chain of Infection





Welcome to Module 2: Chain of Infection. We are glad you're here!


What you learn in this class will help keep you, and your residents, safe. 


Module 2 will present a broad overview of the chain of infection and how pathogens are transmitted.


System requirements

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At the end of this course, you will need to verify that you've fulfilled the course requirements and obtain a learning certificate.


Before we begin...

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Order of Modules

This course is designed to increase your infection control knowledge in long-term care environments. Information from this CBT can be directly applied to your daily activities as a nurse aide.


This course has five modules. You must save or print the certificate at the end of each module. Keep these certificates as proof of completion.

  • Module 1: Introduction to Pathogens
  • Module 2: Chain of Infection/Modes of Transmission
  • Module 3: PPE
  • Module 4: Standard and Transmission-based Precautions
  • Module 5: Cleaning and Disinfection


There is no way to create a lost certificate of completion without taking or retaking the course. If you do not keep a copy for your records, you will need to repeat this course.


Let's get started!


Lesson Objectives


In this CBT, you will:

  • Discuss the links in the chain of infection;
  • Identify how pathogens are transmitted; and
  • Discuss breaking links to the chain of infection.





Chain of Infection Overview

The chain of infection is a set of 6 intertwined links that allow for communicable diseases to spread.


Each step of the chain is required to effectively transmit infectious illness.


Breaking any one of the 6 links can slow the spread of infectious disease.


Chain of Infection


Chain of Infection: Pathogen

Pathogens are microorganisms that cause disease. Without pathogens, we would not have transmissible, infectious disease. Examples of pathogens include:

  • Bacteria
  • Viruses
  • Fungi
  • Parasites


How well a pathogen infects its host has to do with its:

  • Pathogenicity
  • Degree of Virulence
  • Transmissibility


Chain of Infection: Reservoir

A reservoir serves as a place in the environment where a pathogen lives, replicates and thrives.


Areas where a pathogen may live include:


  • Humans
  • Animals or insects
  • Environment


Chain of Infection: Human Reservoirs

In humans, there are two forms of reservoirs: Symptomatic infection and asymptomatic carriers.


Symptomatic infections are more likely to be recognized. This means that the patient's contacts and normal activities will normally be restricted.


Carriers do not display any signs or symptoms of illness. They are asymptomatic but can still spread disease (like Typhoid Mary )


Chain of Infection: Animal & Insect Reservoirs

Any infectious disease that is naturally transmitted from animal to human is considered a zoonotic disease.


Examples of disease spreading from animals or insects to humans include:

  • Lyme disease (ticks)
  • Rabies (animals)
  • Salmonella (raw meats, eggs, and dairy)



Chain of Infection: Environmental Reservoirs

Environmental reservoirs harbor many infectious diseases.


Some examples include:

  • Soil (which acts as a reservoir for Clostridium tetani, the causative agent of tetanus); and
  • Water (which is a reservoir for Legionella pneumophila, the causative agent of Legionnaire's disease.)


Chain of Infection: Portal of Exit


The Portal of Exit refers to any route that the pathogen can leave the reservoir. This depends entirely on the characteristics of the reservoir.


In humans, the main portals of exit include:

  • Alimentary: vomiting, diarrhea, saliva
  • Genitourinary: sexual contact
  • Respiratory: secretions from coughing, sneezing, or talking
  • Skin: open wounds


 Mode of Transmission: Direct Contact

Direct contact is usually considered person-to-person contact. Without physical contact, pathogens that rely on direct contact spread cannot be passed on.


Examples of direct contact include:

  • Skin-to-Skin contact (like touching)
  • Kissing
  • Sexual contact
  • Contact with oral secretions
  • Contact with body lesions


Pathogens that are spread by direct contact include:

  • Gonorrhea
  • Mononucleosis
  • Bacterial Conjunctivitis
  • COVID-19



Mode of Transmission: Droplet

Droplet transmission requires a pathogen to be transferred through the air from its reservoir in a droplet of body secretions. These droplets are relatively large and usually will fall from suspension after 3-6 feet of travel. The droplets will rapidly fall on nearby surfaces (tables, door knobs, telephones, elevator buttons, etc.), and may remain viable. The survivability of the pathogen depends on its type. Some pathogens will live for a few minutes to hours, while others may survive for a few days, outside of a host.



Droplet transmission occurs when a droplet from coughing, sneezing or talking carry the pathogen to the hosts body. The transmission is completed by:

  • Inhaling droplets
  • Droplets entering the mucous membranes of the face
  • A host touching droplets that have settled on surfaces and then touching their face (mouth, eyes, nose).


Examples of illnesses caused by droplet transmission are:

  • Strep Throat
  • Influenza
  • The common cold
  • COVID-19


Mode of Transmission: Blood-Borne

Some pathogens are transmitted directly through blood. These pathogens require that infected blood from the reservoir be directly exposed to the blood of the susceptible host.


This can be accomplished by blood from a needlestick and blood entering mucous membranes or other open wounds.


Examples of blood-borne pathogens include:

  • Human Immunodeficiency Virus (HIV)
  • Hepatitis-B Virus
  • Hepatitis-C Virus


Mode of Transmission: Airborne

Airborne transmission occurs when pathogens smaller than five microns in size remain suspended in the air long after the infected person has left the area. A host then enters the space where the pathogen is suspended and has the potential to become infected.


An example of an airborne illness is measles. Measles can remain suspended in the air for up to 18 hours after the infected individual coughs, sneezes, or talks.


Tuberculosis can remain suspended for up to six hours.


Mode of Transmission: Vector

Vector transmission utilizes insects to transport the pathogen. Insects such as fleas, ticks, and mosquitos are unharmed by the pathogens they potentially carry but can transmit the bacteria or virus when they bite a host.



  • Mosquito: West Nile Virus
  • Fleas: Bubonic Plague (Yersinia pestis)
  • Ticks: Lyme Disease ( Borrelia burgdorferi )



Chain of Infection: Portal of Entry

Opposite the Portal of Exit is the Portal of Entry. This is any route that a pathogen uses to enter the body (host).


Examples include:

  • Inhalation (via the respiratory tract)
  • Absorption (via mucous membranes such as the eyes)
  • Ingestion (via the gastrointestinal tract)
  • Inoculation (as the result of an inoculation injury)
  • Introduction (via the insertion of medical devices)


Chain of Infection: Susceptible Host

The last link in the chain of infection is the susceptible host. This is the organism (e.g., You or your resident!) that will feel the effects of the infectious disease that has traveled through the chain of infection.



How susceptible a host may be, depends on many factors:

  • Age
    • The very young or very old are usually more susceptible.
  • Health status
    • Malnourished, dehydrated, or otherwise unhealthy persons are more at risk
  • Medication usage
    • Immune suppressing drugs allow pathogens to take hold more freely
  • General resistance factors
    • Intact mucous membranes and skin, and robust cough and sneeze reflexes help defend against invading pathogens.


Breaking the Chain

To break the chain of infection, and stop infectious disease spread, interventions can be directed at:


  • Controlling or eliminating agent at source of transmission
  • Protecting portals of entry
  • Increasing host's defenses


Targeting one or more of these areas can help to slow or stop the spread of infectious disease.


Breaking the Chain: Controlling the Source of Transmission

Stopping disease spread at the source may be an appropriate intervention for many pathogens found in long-term care facilities.


The first step at breaking the chain is to identify the offending pathogen. This will allow for treatment protocols specific to the disease-causing microorganism.


Examples include:

  • A patient with a bacterial infection, like strep throat, can be treated with antibiotics to clear the infection.
  • A patient who is an asymptomatic carrier may also be treated to clear the infection and prevent spread to others.
  • Cooking food to safe temperatures ensures that all bacteria and/or spores have been killed before consumption.



Breaking the Chain: Protect Portals of Entry

Other interventions protect portals of entry. There are many ways that targeting the portal of entry can help to prevent the spread of communicable disease.


Examples include:

  • Wearing proper PPE when you are knowingly exposed to a pathogen
    • Masks, gloves, face shields, etc.
  • Practicing hand hygiene consistently.
    • Handwashing and using alcohol-based hand rubs
  • Isolating those who are infectious and minimizing contact
  • Filtering or changing the flow of air
    • Negative pressure rooms





Breaking the Chain: Increasing the Host's Defense

Increasing the host's defense allows for the body's immune system to fend off attacking pathogens before the person ever knows they have been exposed.


Vaccination programs help the body to create antibodies to targeted pathogens before exposure.


Prophylactic drug use for high-risk patients can help to mitigate the spread of disease by preventing a pathogen from gaining ground over the immune system.


Some interventions attempt to prevent a pathogen from encountering a susceptible host.


Herd immunity  suggests that if a high enough percentage of individuals in a population are resistant to a pathogen, then those few who are susceptible will be protected by the resistant majority.


The theory is that the pathogen will not have a sufficient number of reservoirs available to survive until it finds a susceptible host.



Objectives Review

So far in module 2, we have:

  • Discussed the links in the chain of infection;
  • Identified how pathogens are transmitted; and
  • Discussed breaking links to the chain of infection.


The following slides will include a short concept check to help you demonstrate your understanding of the content provided.





You have successfully completed Module 2: Chain of Infection


Please review your score.


Save or print your certificate for your records.


Remember, there is no way to create a lost certificate of completion without retaking the course. If you do not keep a copy for your records, you will need to repeat this course.




Next up:



Module 3: PPE