The cycle of infection
As a medical assistant, your role in helping to create and maintain a safe and healthy environment for both patients and employees is key. This role includes understanding how infections occur and are transmitted in the population and practicing all necessary infection control precautions. To understand how infections are spread, you need to understand the cycle of infection.
The cycle of infection
Disease is defined as a specific illness with a recognizable group of signs and symptoms and a clear cause (e.g., infection, environment). We recognize and categorize many types of diseases: hereditary (genetic), drug induced, autoimmune, degenerative, communicable, and infectious, to name only a few. Sometimes a specific disease may fit two or more categories.
Any disease caused by the growth of pathogenic microorganisms in the body falls into the category of infectious diseases. The entrance of a living microbe into the body is not a disease until the infected cell or individual shows harmful changes in structure, function, or biochemistry. In fact, a pathogen may be ingested, injected, or inhaled and never cause disease. However, an unaffected person can still transmit the infection to another person. In this case, we call the unaffected person a carrier.
Microorganisms are almost everywhere. We carry them on our skin, in our bodies, and on our clothing. They can be in ice, boiling water, the soil, and the air. The only places free of microorganisms are certain internal body organs and tissues and sterilized medical equipment and supplies. In the normal state, organs and tissues that do not connect with the outside by means of mucus-lined membranes are free of all living microorganisms.
Certain factors are required for an infectious disease to spread. These factors, or links, make up the chain of infection. Break the chain, and you break the infection process.
The chain of infection starts with the infectious agent. Infection cannot occur without the presence of an infectious microorganism. The best way for healthcare workers to prevent the spread of disease is to use infection control procedures, such as the following:
- Consistent hand washing
- Proper use of antiseptics
- Effective disinfection and sterilization methods
Six links make up the cycle of infection. These six parts all must be present for infection to occur:
- Infectious agent
- Reservoir host
- Means of exit
- Means of transmission
- Means of entrance
- Susceptible host
Reservoir host
The second link in the chain of infection is the reservoir. The most common reservoirs are people, insects, animals, water, food, and contaminated instruments and equipment. Most pathogens must gain entrance into a host, or else they die. The reservoir host supplies all of the conditions needed for microbial growth, allowing the microbes to multiply. The pathogen either causes infection in the host or, in the case of vector-borne diseases, exits the host in great enough numbers to cause disease in another host.
Sanitization, disinfection, and sterilization are key to breaking the chain of infection at the reservoir link. Pest control can also help with the insect and animal components.
Means of exit
The chain of infection continues with the means, or portal, of exit - that is, how the pathogen escapes the reservoir host through the mouth, eyes, intestines, reproductive tract, nose, ears, urinary tract, or open wounds.
The use of standard precautions (e.g., gloves, masks, proper wound care, correct disposal of contaminated products, and handwashing) helps control the ability of infectious material to spread from one host to another.
Taking measures to control the splatter of body fluids - whether from a cough, a sneeze, or during a procedure - is another important way to break this link.
Means of transmission
After exiting the reservoir host, the organism needs a mode of transmission.
Direct transmission occurs from contact with an infected person, with discharges from an infected person (e.g., feces or urine), or with infected soil.
Direct transmission includes droplet transmission. This refers to the droplets that come from sneezing, coughing, or even talking. Those droplets land on the next potential host.
Indirect transmission refers to the transfer of microorganisms by suspended particles in the air, inanimate objects, or animals and insects. When microorganisms are suspended particles, this is considered airborne transmission. Transmission occurs through inhalation of the microorganisms.
Vectors are animals or insects that harbor pathogens and can transmit them. Transmission can also occur through contaminated food or drink or contact with contaminated objects (called fomites).
Proper sanitation of water and food; the use of sanitization, disinfection, and sterilization procedures; and the use of germicides (e.g., Wavicide and Cidex) all help to control the transmission of pathogens.
Means of entry
The next link in the chain of infection is the means, or portal, of entry. This is how the transmitted pathogen enters a new host through the mouth, eyes, intestines, reproductive tract, nose, ears, urinary tract, or open wounds.
As with the portal of exit, the use of standard precautions helps to prevent the infectious agent from entering the body.
Providing proper first aid and good personal hygiene also play a role in breaking this link.
Susceptible host
The final link in the chain of infection is exposure of the pathogen to a susceptible host. If the host is susceptible (i.e., capable of supporting the growth of the infecting organism), the organism multiplies.
Various factors contribute to a host’s susceptibility:
- Genetic factors
- Quantity of organisms
- State of the individual’s physical health
- Stress
- Poor health
- Poor nutrition
- Poor hygiene or crowded living conditions
If conditions are right, the organism reaches infectious levels, and the susceptible host can start the chain of infection all over again.