6.1: Infection Cycle
By the end of this section, you will be able to:
- Describe the infection cycle
- Identify the different stages of infection
- Explain the two responses the body has as defense mechanisms against infection
Nurses are on the front lines of infection control , which is the discipline of stopping or preventing the spread of infectious agents, and play an essential role in the prevention of infectious diseases. It is crucial for nurses to understand how infectious diseases spread to ensure vigilance and perform proactive initiatives to prevent and control infections. Microorganisms play a major role in the transmission of diseases. By understanding the conditions that foster the spread of infection, nurses can implement evidence-based interventions to break the cycle and stop the chain of infection . The process of infection control includes handwashing, environmental sanitizing, proper waste management, and adherence to isolation precautions. The main goal of infection prevention for nurses is to prevent the transmission of diseases. Nurses must understand how infections occur and how infection-prevention protocols work to prevent such infections. In doing so, they will ultimately protect themselves and their patients against exposure to infectious agents. Such knowledge includes an understanding of the infection cycle, how and why infections manifest, use of personal protective equipment (PPE) and sterile technique , and the most effective ways to control infections. While proper handwashing is a critical component of infection prevention, there are a number of other ways that nurses can stem the spread of infection and protect themselves and their patients.
An infection that has developed within a healthcare setting is called a healthcare-associated infection (HAI) . HAIs can develop from contact within the healthcare setting or as a result of healthcare interventions that take place outside of a healthcare setting. HAIs can spread rapidly and are a serious threat to nurses, patients, families, and the overall hospital system. Patients who develop any infection are at risk for prolonged hospital stays, long-term complications, and/or death, but HAIs are specifically dangerous as they are typically resistant to bacterial treatment and spread rapidly to often immunocompromised hospitalized patients. In the 2021 National and State Healthcare-Associated Infections Progress Report, the Centers for Disease Control and Prevention (CDC) found that each day approximately one in thirty-one U.S. hospital patients will contract an infection associated with their health care (CDC, 2022).
The Infection Cycle
In order for an infection to spread from one individual to another and cause disease, six specific phases must occur. This process is known as the chain of infection , and it only results in infection if all six links of the chain are present and intact (Figure 6.2). The six links are a causative agent, a source, a means of exit from the body, a method of spread, a way into the body, and a susceptible host . This chain can occur from a direct transmission between a current and future host or through a more complex pathway where transmission results from multiple intermediate hosts. If, at any time, one of the links breaks, the spread of the infection will halt. In order to break the chain, it is important for nurses to understand how the sequence and function of each link in the chain operates.
Causative Agent: Microorganisms
Single-celled, microscopic organisms called microorganisms are too small to be seen by the naked eye. Their presence as the “causative agent” is the first step in the chain of infection. The term microorganism encompasses different life-forms with individual and unique sizes and characteristics. The most common types of microorganisms are bacteria , viruses, and fungi. They are found in all elements of life, including water, soil, air, and the human body. Not all microorganisms that live on the human body cause negative outcomes, and certain bacteria are beneficial to human health and well-being. Still, certain microorganisms can cause severe infection, spoil food, and destroy other materials.
Normal Flora
The community of microorganisms that can live on another living organism or inanimate object without causing any diseases or complications is called normal flora . From the moment of birth, humans are colonized with normal flora by passage through the birth canal. This normal flora helps to prevent individuals from becoming colonized with more dangerous bacteria, which could result in infection. There are two groups of normal flora.
- Resident flora is predominantly present in a particular area of the body and cannot typically be removed with standard hand hygiene. When disturbed, it re-establishes itself harmlessly in deep layers of skin.
- Transient flora includes microorganisms that are acquired by contact with objects or another person. These microbes can be nonpathogenic or pathogenic. Handwashing is effective in removing these microbes.
Infectious Agent
A pathogen is defined as any type of microorganism that causes disease to its host. Pathogens are also referred to as infectious agents, because they cause infections within the body. Pathogens are comprised of viruses, bacteria, fungi, protozoa, worms, or prions. Temperature, moisture, pH levels, oxygen, and access to water are all factors that contribute to a pathogen’s ability to grow.
Bacteria are single-celled organisms that can live in or on people’s bodies. Certain bacteria are beneficial to humans and can help digest food or enhance the immune system. However, when bacteria cause an infection, antibiotics can be used to either kill the bacteria or prevent their multiplication. Viruses, by contrast, do not have cells of their own; they are built from short sequences of either DNA or RNA that are required for the virus to reproduce. Viruses invade healthy cells that they establish as a host and then begin to multiply from within those cells. Their mode of replication occurs as a burst of thousands of particles from a single virus over a short period of time. Outside of a healthy host cell, viruses are dormant and unable to reproduce due to a lack of materials. Antibiotics do not work to kill viruses, but antivirals may be available to lessen the severity of symptoms. Treatments such as antipyretics, throat lozenges, and saline spray can be provided to support symptom management and support the patient's immune system as they work to fight an active infection. Vaccinations are an excellent example of a tool available to help prepare the immune system to recognize and fight a viral infection by providing passive immunity.
Normal flora can become a pathogen if it presents itself in a region of the body where it is not typically found. For example, multiple bacteria found in the bowels are harmless within that environment, but they can cause an infection if present in the urinary tract. The body’s extreme response to widespread infection, called sepsis , is the outcome of an inappropriate immune response to an infection that results in function failure in multiple organ systems within the body. Severe complications to organs may occur if untreated, which can ultimately lead to death.
Reservoir
All infectious disease agents require a host species to flourish. The reservoir is the habitat or source of the pathogen. The reservoir can be viewed as the pathogen’s home, providing a place for the pathogen to survive, grow, and multiply. Most pathogens thrive within a warm, moist, and dark environment. This is why the human body is the most common reservoir for pathogens. Additionally, animals, insects, food, water, and environmental surfaces can all be reservoirs for pathogens.
Human reservoirs may or may not show signs and symptoms of infection or illness. A carrier is a person who does not display any signs of infection but can still transmit the pathogen to other people. Asymptomatic carriers have the pathogen, but do not display any symptoms. Many times, they do not know they have the pathogen and unintentionally contribute to the spread of infection throughout a given population. But pathogens do not only travel from human carriers to other humans. People can also become infected from pathogens that have animal reservoirs. Most of these pathogens are transmitted between animals, but a human may become an accidental host. The term zoonosis refers to an infectious disease that can naturally transmit from animal to human. Common examples of zoonotic diseases include Yersinia pestis from rodents, Bacillus anthracis from sheep, ZIKA from mosquitos, and Flaviviridae from birds. Environmental matter, such as water and soil, can also act as a reservoir for certain infectious agents. The agents that cause tetanus ( Clostridium tetani ) and botulism ( Clostridium botulinum ) can survive for years within soil and remain infectious for humans. So, whether a pathogen comes from a human or animal reservoir or from environmental matter, it can still cause disease once it is transmitted.
Portal of Exit
For an infection to spread, a pathogen must leave its existing reservoir. The portal of exit is the path by which the pathogen leaves the reservoir; in the case of humans, the most frequent route is through bodily fluids or coughing/sneezing. The body’s natural response is to remove a pathogen and attempt to expel it. The portal of exit usually corresponds to the localized site of the pathogen. Examples of this include the influenza virus, which exits the respiratory tract through coughing and sneezing, or Clostridioides difficile in the gastrointestinal tract, which exits through stool. Broken skin—such as wounds, abrasions, bites—can serve as a portal of exit for pathogens through blood and purulent drainage, which is commonly known as pus, and appears as thick white, yellow, or brown fluid. Blood-borne pathogens can transmit from mother to fetus by crossing the placenta.
Methods of Transmission
The method that a pathogen uses to spread from one host to another is called transmission . The most frequent mode of transmission of pathogens is through contact, either direct or indirect (Figure 6.3).
- A process called direct transmission occurs when a pathogen transfers directly from an infected person. The pathogen can be passed from person to person through direct transmission. This mode of transmission can occur during any physical contact with a patient, including activities such as bathing, changing dressings, drawing blood, turning, and activities of daily living.
- A process called indirect transmission occurs when a pathogen is spread to a new host through an intermediary such as the air, food, water, animals, or objects. Certain pathogens can live only a few minutes outside of a host while others can live for years in the proper environment. Indirect transmission can occur in a hospital from, for example, ineffective hand hygiene, improper cleaning of medical equipment, and failure to change gloves between patients. Other forms of indirect transfer from within a hospital include equipment that is transferred from one patient’s room to another, such as medication carts, vitals machines, and glucose monitors. These items require extra care when cleaning.
- A process called droplet transmission occurs when a pathogen travels through a spray of water droplets that are released when an infected person coughs, sneezes, or talks. These droplets are typically inhaled through the nose, mouth, or eyes. Due to their larger size (> 5 µm), droplets are propelled only a short distance through the air and do not remain suspended, so droplet spread is classified as direct transmission and does not require an intermediary. Examples of diseases that transmit through droplets include influenza, rubella, pertussis, and meningococcal infection.
- A process called airborne transmission occurs when pathogens are carried by dust or the nuclei of an evaporated droplet and remain suspended in the air. Because of their small size (5 µm or smaller), these nuclei can remain suspended in the air for long durations of time, float considerable distances, and potentially infect large groups of people. For instance, SARS-CoV-2 coronavirus transmission can occur in a room in which an infected person had previously been, because the virus remains suspended in the air. Because airborne transmission occurs through the inhalation of the pathogen by a susceptible host, healthcare facilities need to put into place special air-handling processes, such as negative pressure, to prevent infection.
- A process called vector transmission occurs when blood-feeding arthropods infect animals or humans. Examples of blood-feeding arthropods are fleas, ticks, and mosquitos. Commonly known vector-borne diseases include malaria, Lyme disease, and West Nile virus.
Portal of Entry
The site through which a pathogen enters the susceptible host is called the portal of entry (Table 6.1). Commonly, pathogens enter a new host using the same portal of exit utilized to leave the reservoir . For example, if the pathogen is transmitted from the respiratory tract through a sneeze or cough, then the portal of entry would also be the respiratory tract of the new host from inhalation of the droplets or touching a surface contaminated with the droplet and touching a mucous membrane . In healthcare settings, wounds, surgical sites, intravenous access sites, and indwelling catheters can all provide a portal of entry for pathogens.
| Portal of Entry | Description |
|---|---|
| Mucosal | Through the eyes or nose |
| Respiratory | Through the respiratory tract |
| Genitourinary | Through the urinary tract |
| Cutaneous | Through wounds or abrasions |
| Gastrointestinal | Though the intestinal tract |
Susceptible Host
The final link in the chain of infection is a susceptible host , the organism or person at risk for infection. The degree to which a host is at risk is dependent on their immunity and ability to resist or limit susceptibility. A host may have specific immunity to a particular pathogen through protective antibodies. The antibodies may have developed as a response to a previous infection, toxin, or vaccine. Factors that increase susceptibility include age, chronic illnesses, a compromised immune system, or immune deficiency.
Age is a nonmodifiable risk factor. As adults age, their functional immunity declines, which increases their susceptibility to infection. Older adults are more prone to developing an infection due to multiple factors: the immune system no longer functions as optimally or vigorously; they may experience cognitive impairments, which could make them less likely to comply with necessary hygiene practices; and they are more likely to be diagnosed with comorbidities, such as diabetes, heart failure, or rental insufficiency, all of which can affect the body’s ability to fight infections.
As individuals age, their lifestyles typically change as well. Nutritional intake can decrease, reducing protein, vitamins, and electrolytes. This can lead to a decrease in body mass, which increases susceptibility to infections.
The National Institute on Aging recommends vaccination as the number one way to prevent infections in an aging population. The Institute also seeks to reduce the number of infections by promoting healthy aging. This entails remaining active and continuing modified physical activity, enjoying proper nutrition, and maintaining regular and routine appointments with health providers (National Institute on Aging, 2022).
Stages of Infection
All infections progress through a predictable course of four stages (Figure 6.4). Each pathogen can produce distinct and diverse symptoms. An individual’s immune response to the pathogen will determine the length and intensity of each stage and account for variability seen between one individual and another.
Incubation Period
The stage of infection known as the incubation period begins once a pathogen successfully enters a new host. During the incubation period, a person does not show signs and symptoms of an infection because there are not enough pathogens to cause symptoms. The person therefore does not suspect they have been infected. One example is the common cold. The patient feels healthy for one to three days, but then starts to demonstrate symptoms of illness once the level of pathogen has increased in their system. Despite being asymptomatic those first few days, pathogens are continually multiplying and can still be spread to other hosts. Because of this, the longer the incubation period is, the more likely a person is to be unknowingly spreading infection to other people. The incubation period can vary in time from as little as one day, as with the influenza virus, to two to three months, as with the hepatitis B virus. It can even last years, as with the human immunodeficiency virus.
Prodromal Stage
The stage of infection called the prodromal period begins at the initial appearance of mild or vague symptoms. These symptoms arise as a result of activation of the immune system and typically present as fever , pain, soreness, or inflammation. Symptoms at this stage are often too general to indicate a specific disease. Referring to the patient exposed to the common cold, after one to three days of feeling healthy, they may then have symptoms such as a headache, scratchy throat, and watery eyes. Though present, these symptoms are nonspecific and could be associated with a variety of other illnesses. This stage can vary in its duration, but it is typically shorter than the incubation period.
Illness
The illness period stage of infection begins when a person experiences the specific signs and symptoms of a certain disease. This period represents the peak of the infection, and it is during this time that a person is highly contagious. In this stage, our patient who was exposed to the common cold will start to show specific symptoms such as mild hacking cough, sneezing, achy muscles and bones, and low grade fever. If the individual’s immune system (with or without medical intervention) is able to combat the pathogens, a period of decline in the pathogens begins. The infection begins to weaken, and symptoms decrease.
Convalescent Period
The final stage of infection is the convalescent period . This is the stage where recovery and healing begin. Closing out our example of the patient who has the common cold, the patient’s symptoms will fade, and the patient will report feeling back to baseline. During this time, a person is gradually able to return to their normal functions; some infections, however, can result in permanent damage from which the body is unable to repair and recover.
Body’s Natural Defense Mechanism Against Infection
The human body’s immune system provides a mechanism for staying healthy through protection against harmful pathogens. An immune response can be classified as either nonspecific, meaning it targets pathogens in a nonspecific, less effective manner, or specific, which allows for a high level of adaptation and effectiveness. Nonspecific immunity includes physical, chemical, and cellular defenses that are classified as either primary or secondary. Secondary defenses are broken down further into inflammatory and immune responses. Specific defense would be antibodies from the immune system targeting a specific antigen that they are designed to identify and destroy.
Primary Defenses
Primary defense barriers prevent pathogens from entering the body through structural barriers, destroy them once they have entered the body, or flush them out of the body (Table 6.2). These barriers are not triggered as a response to pathogens but instead serve as a continuous first line of defense against infection.
| Defense | Description |
|---|---|
| Skin | The skin provides a highly effective physical barrier to infection. The top layer of skin, the epidermis, is made up of cells containing keratin, which makes the skin surface mechanically tough. If skin integrity is compromised due to injury, such as abrasions, cuts, incisions, or burns, the barrier is breached, which creates a portal of entry. |
| Mucous membranes | The mucous membranes that line the nose, mouth, lungs, and digestive and urinary tracts are coated with secretions that aid in fighting against potential pathogens. The nares, trachea, and bronchi are coated with mucous membranes that trap pathogens. Coughing and sneezing allow pathogens to be forcibly expelled from the body. |
| Stomach | The acidic environment of the stomach destroys pathogens that enter the digestive tract. Normal peristalsis—the muscles moving food through the digestive tract—as well as vomiting and diarrhea work to remove pathogens that enter the tract. Additionally, the natural flora of the body, specifically in the gastrointestinal system, serves as a defense mechanism. |
| Eyelashes and eyelids | These structures provide a physical and mechanical barrier from dust and airborne microorganisms through blinking; tears wash away organisms. |
| Cilia | Housed in the nares, cilia move a layer of mucus that covers the airways. This mucus traps pathogens, preventing them from reaching the lungs. |
Inflammatory Response
Pathogens that are not stopped by primary mechanisms and are able to enter the body trigger a second set of defenses. Nonspecific, innate immune responses work to recognize and eliminate pathogens.
One of the first responses that occurs when a pathogen breaches the nonspecific innate immune system is an inflammatory response . This response can entail an area of the body by swelling, turning red, feeling hot, having pain, or losing function. Although inflammation is often perceived as a negative consequence that results from injury, it actually establishes a physical barrier against infection. The process of inflammation aids in the recruitment of cellular defenses, which remove pathogens and damaged cells while initiating repair mechanisms.
The process of inflammation is triggered when damaged cells release histamines and other chemicals. The rise in histamines increases the permeability of the blood vessels, which results in additional blood flow to the area. This additional blood flow manifests as localized warmth and redness. Because the blood vessels are more permeable, fluid leaks from them and accumulates in surrounding tissue, resulting in swelling at the site. The swelling places pressure on nerve endings, resulting in pain.
The cognitive skill of recognizing cues requires the nurse to collect patient data from health assessments, the environment, and health records, and identify abnormal findings.
A nurse assessing a patient who has a suspected infection will consider findings related to an inflammatory process, such as increased body temperature, heat at the site, redness, or swelling. Additional assessment data gathered should include physical history, degree of pain, loss of function, and lab levels that indicate infection, such as an increased white blood cell count.
A fever can be one of the body’s responses to pathogens that cause inflammation; it is defined as a rise in core body temperature and is a component of the inflammatory response extended past the localized site. A low-grade fever is a natural immune response and defense mechanism; many providers will not initiate pharmacological interventions until it rises above 102°F (38.9°C). After all, a fever enhances the nonspecific immune defenses by stimulating an increase in white blood cells (WBCs). WBCs are produced in the bone marrow and are an essential part of the immune system as their function is to find, fight, and destroy infection within the body. If WBCs are elevated, this can indicate an immune response from the body and demonstrate actively fighting a disease. The rise in temperature can also prevent the growth of many pathogens and can trigger specific immune responses.
Immune Response
In contrast to a nonspecific immune response, specific acquired immunity occurs when an individual’s immune system acquires antibodies from a different source. In other words, this type of immunity provides a targeted response to a specific pathogen and can be acquired either actively or passively. The main function of the immune system is to recognize self from nonself and initiate a response accordingly. An antigen is anything the immune system recognizes as a foreign object or substance and subsequently initiates formation of antibodies. Immunoglobulins, or antibodies , are proteins created in the body in response to an antigen in order to fight the identified substance or toxin.
When a pathogen enters the body and active immunity occurs, antibodies form to help protect the body from that pathogen. In subsequent invasions from that same pathogen, the body is able to respond rapidly to the antigen. Active immunity can be acquired through infection, such as with chicken pox, or artificially, through immunizations. The result of antibodies being passed from one person to another is passive immunity . This can occur naturally through the placenta or breastfeeding, or it can occur artificially through injections of serums or blood products that contain antibodies.
As part of its mission to promote health and prevent disease, the CDC publishes written recommendations for vaccinating U.S. children and adults.