The Weakest Link! Breaking the Chain of Infection

Author: 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, “You’re 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.

Infectious Agent

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 patient’s 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 patient’s 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 the reservoir.

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 patient’s 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 host’s 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 throat!

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 system’s 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 body’s 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 body’s 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 quickly.

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 to invade.

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 will develop.

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 agency’s policies for responding to and handling infectious diseases will reduce your chances of becoming ill from exposure to an infectious agent.

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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.