The Weakest Link! Breaking the Chain of Infection
Robert G Nixon
Several years ago, while sitting among family members at a holiday gathering,
the conversation turned to prehospital care and the nature of the job. As the
discussion continued, the topic turned to HIV and AIDS. Immediately after mentioning
that I had provided care for several acutely ill patients with AIDS, some of
the group looked at me with horror a look that said, Youre
infectious and can give the disease to me. One man appeared to be on the
verge of hysteria. It was obvious that he had no clue about infectious diseases.
Perhaps a lot of misinformation still exists among prehospital providers and
the general public about how a disease is transmitted.
In many initial and continuing education programs, there is a wealth of information
about certain infectious diseases from hepatitis to HIV to STDs. Oftentimes,
there will be some discussion pertaining to the basics of infectious disease
including mode of transmission, portal of entry, the immune system, and similar
topics. One of the aspects of infectious diseases that is often omitted includes
a discussion of the critical components of the Chain of Infection. People tend
to forget that in order to be exposed to and contract a communicable disease
each component of the Chain of Infection must be intact. If any link in that
chain is broken, there is no infection.
Links of the Chain
There are six links in the Chain of Infection. Each link must be intact in
order for a disease to be transmitted and infect another person. The links of
the chain include Infectious Agent, Reservoir, Portal of Exit, Mode of Transmission,
Portal of Entry, and Susceptibility. The infectious disease process can be broken
at any one of these links, preventing infection in another person. To understand
how the concept of the weakest link works, let us examine each link
in the Chain.
In order for an infection to occur there must be an infectious agent. In essence,
no bugs no disease. For example, consider walking near someone who sneezes
as you pass by. Are you exposed to a communicable disease? If the person who
sneezes did so because of some external irritant such as pepper entering the
nose, there is no infectious agent. Without an infectious agent, you cannot
become ill. In another example, consider you encounter a patient who has partied
a bit too hearty. While assessing his condition, the patient proceeds to vomit
on your shirt and the stench of an alcoholic beverage now permeates your clothing.
Have you been exposed to a communicable disease? Not unless there is an infectious
viral or bacterial agent present in the patients stomach or the patient
has gastric bleeding.
As the situation progresses, your partner begins to vomit. Was your partner
exposed to an infectious disease from the patient? Most likely, your partner
is having a sympathy reaction to the patients vomiting.
Disinfecting surfaces such as counters or equipment with a specific cleanser
can also destroy infectious agents. Without an infectious agent, there is no
Chain of Infection.
To be considered infectious, there must be a sufficient quantity of bugs to
cause an infection. One bacterium or virus will generally not cause an infection.
Further, these bugs need to be concentrated in a specific location such as the
airways, lungs, blood, or other body area. But, the body is not the only reservoir
that can harbor these bugs. Consider the sharps container or any piece of soiled
equipment. Once a needle has been used, it is placed into a sealed sharps container.
This container is a reservoir for infectious diseases and, if opened, could
pose a serious threat to the employees.
Not only is the sharps container a reservoir, so is dried blood. While drying
destroys many infectious agents, some organisms are not affected by drying.
Some bacteria form spores while viruses merely become inactive. When the circumstances
are better, the bacteria and viruses reactivate and infect. Cleaning contaminated
equipment does two things it destroys the infectious agent and eliminates
Portal of Exit
From the reservoir, the bugs need a way out of the body. The passage out of
the body can include the nose, mouth, open injury, or any other way to leave
the reservoir. If an infectious agent is trapped inside the reservoir and cannot
leave, there is no risk of infection. For example, think about that sharps container
mentioned earlier. There is an infectious agent and reservoir, but because the
container is sealed, there is no way for the agent to get out.
In another example, consider drawing a vial of blood from a patient with Hepatitis
B for testing at the hospital. You have drawn the blood that is in a sealed
container. Is there an infectious agent? Yes. Is there a reservoir? Yes. Is
there a portal of exit? No, the vial is sealed. Because the vial is sealed,
the Chain of Infection is broken.
Mode of Transmission
The infectious agent must have a way from its reservoir to the new host. In
thinking about the vial of blood above, if the cap to the vial was removed,
there would be a portal of exit, but the blood must still get from the vial
onto you. If the blood spills onto your clothing or hands, there is a mode of
transmission. In another example, consider a person with the flu and chest congestion.
The infectious agent is the flu virus, the reservoir is the patients lungs
and airways, and the portal of exit is the mouth and nose. When coughing, the
patient expels infectious fluids from the airways into the air. This creates
a droplet mode of transmission that transports the agent to the new host. Can
this mode of transmitting the flu virus be blocked? Covering the mouth when
coughing is a courtesy to those nearby and reduces, but does not eliminate the
risk of transmitting the illness.
Portal of Entry
Once the infectious agent has found its way out of the reservoir, it must have
a way into the new host. This portal of entry can be the new hosts nose,
mouth, airways, eyes, gastrointestinal system, or skin. Each of these areas
of the body can permit the entry of invading bacteria or viruses. Yet, each
of these areas has natural barriers against invasion. The nose and airways are
lined with mucus-producing tissue containing hair-like fibers called cilia.
This tissue traps larger particles of dust and contaminants, preventing entry
into the body. To clear the nose, a person either sneezes or blows the nose
to remove the trapped debris.
The smaller airways are also lined with tissue that contain cilia and produce
mucus. Smaller particles are trapped in the lower airways and, by the action
of the cilia, are swept up and out of the airways. On reaching the throat, the
mucus is coughed and either spit out or swallowed. Think about clearing your
Infectious agents contacting the eyes may be washed away by tears. However,
some of those bug-containing tears may be rinsed into the tear ducts and into
the nose. It is important not to rub your eyes without washing your hands.
The mouth contains enzymes in the saliva that begin to destroy the bacteria
or viruses as they enter the body. The stomach acid may destroy the bugs that
survive the mouth before entering the intestines. However, some bacteria may
survive the mouth and stomach, and enter the small intestines. The intestines,
through mucus production and peristalsis help block the bacteria or viruses
from entering invading the lining of the intestines and getting deeper inside
the body. Think about food poisoning and what happens after the bacteria enter
the gastrointestinal system. The stomach goes into reverse and vomiting
begins. This helps eliminate the infectious agent from the system. The intestines
move the infectious material though the system quickly, which may produce the
cramping sensations. Mucus production along with reduced water absorption in
the intestines rinses the bacteria from the system and causes diarrhea.
The skin has natural barriers including oils containing fatty acids that tend
to destroy infectious agents or make the skin a less hospitable environment.
Additionally, the outermost layer of the skin, epidermis consists of dead cells
that slough away with washing. Washing away these dead cells also washes away
the infectious agent.
In addition to our natural barriers, we use other barriers that prevent an
infectious agent from entering the body. Universal precautions are designed
specifically for this purpose. Latex gloves prevent direct contact of an infectious
agent with the skin. A face mask, goggles, and gown also prevent contact of
the agent with the skin. In some cases, a respirator is used as a barrier against
the infectious agent. A respirator is worn when in contact with a tuberculosis
patient and is designed to filter the small bacterial particles from the air
so that the bugs are not inhaled.
Again, there must be a portal of entry into the body for the risk of infection
to be present. If the portal of entry is blocked, the Chain of Infection is
broken and there is no risk.
In order for an infectious agent to cause an infection, the last link of the
Chain, the new host must be susceptible to the disease. Susceptibility is affected
by a number of factors including number of organisms and the immune systems
status against the microbes. The number of organisms is important. One or two
bacteria or viruses are generally not sufficient to cause infection. While some
infections can be caused by as few as 10 organisms, most infections require
hundreds to thousands for an infection to develop.
The bodys immune system also plays a role in susceptibility. While the
components of the immune system are too detailed to discuss in this article,
there are a couple of concepts to know. First, when an infectious agent enters
the body it is not recognized by the body, which does not know how to fight
the invasion. The bodys immune system does, however, act to produce antibodies
that will respond to the agent and destroy it in the future. For example, as
a child you may have had the chickenpox. When you first contracted the illness,
the body did not recognize the virus and did not know how to fight it. Your
body did develop antibodies to the virus and, when the virus tries to invade
your body in subsequent years, your immune system recognizes and destroys it
Vaccinations work the same way. By getting a vaccination against a particular
illness, you stimulate the immune system to fight the invading bug. Because
the body can successfully defend against the organism, you are not susceptible
to the illness.
The body needs to be working at peak performance. Stress, fatigue, and inadequate
nutrition can reduce the effectiveness of the immune system and increase the
susceptibility to disease. Getting rest, avoiding stress, and eating properly
will enhance the immune systems ability to fight viruses or bacteria that try
As with other links in the chain, if you are not susceptible to the infectious
agent, the Chain of Infection is broken and you will not become ill.
An Infection Control Program is designed to reduce the risk of exposure to
bloodborne or airborne diseases through the use of Universal Precautions as
well as engineering control designed to reduce the chance that an infectious
agent will pose a threat to the employees. These universal precautions and other
policies and procedures are based on the Chain of Infection concept. When you
find the 'Weakest Link' and break the chain, there is no risk that an infection
This article has addressed the six components of the Chain of Infection. Each
link in the chain, Infectious Agent, Reservoir, Portal of Exit, Mode of Transmission,
Portal of Entry, and Susceptibility must be in place for you to be at risk for
contracting a communicable disease. Implementing universal precautions and following
your EMS agencys policies for responding to and handling infectious diseases
will reduce your chances of becoming ill from exposure to an infectious agent.
Feedback? We'd love to hear from you!
About the Author: Bob Nixon is a paramedic and has been involved in EMS
since 1971. He has taught initial and continuing education courses since 1974.
He has served in both the private and public sector EMS. Bob has also served on
the Editorial Advisory Board for Emergency Medical Services magazine. He has authored
approximately 30 articles and several textbooks for EMTs and paramedics.