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3. Chain of Infection
Certain conditions must be met in order for a microbe or infectious disease to be spread from person to person. This process, called the chain of infection, can only occur when all six links in the chain are intact. By breaking this chain at any of the links, the spread of infection is stopped.
Links in the chain
• Disease Microorganisms (Agent). These are the pathogens that cause
communicable diseases. Most commonly these are bacteria, virus, fungi or parasites.
• Reservoir. The reservoir (source) is a host which allows the pathogen to live,
and possibly grow, and multiply. Humans, animals and the environment can all be reservoirs for microorganisms. Sometimes a person may have a disease but is not symptomatic or ill. This type of person is a carrier and she/he may be referred to as ‘colonized’. Examples of reservoirs are standing water, a person with a common cold or syphilis, or a dog with rabies.
• Mode of Escape. This refers to the route by which the infectious
microorganisms escape or leave the reservoir. For example, pathogens that cause respiratory diseases usually escape through the respiratory tract (coughing and sneezing).
Diagram: The chain of infection
Modes of Escape Breaking the Link
Respiratory Tract. Microorganisms leave
the body of the infected person by means of droplets exhaled as a spray when coughing, sneezing, talking, singing or just breathing. Microorganisms also escape through nose and throat secretions.
- Wear a mask
- Do not talk directly into patient’s face
- Stay home if you are sick
- Practice good cough etiquette (cover your coughs and sneezes)
- Perform good hand hygiene
Gastrointestinal Tract. Microorganisms
that leave the body of the infected person by means of body secretions (e.g. stool and vomit). For example hepatitis A virus is shed in the stool of the infected person.
- Handle and dispose of body secretions properly
- Use personal protective equipment
- Perform good housekeeping
- Perform good hand hygiene
Skin. Microorganisms that leave the body of
the infected person by wound drainage or through skin lesions.
- Dispose of wound dressings properly
- Use personal protective equipment (PPE)
- Perform good hand hygiene
Blood. Infection may occur when someone’s
blood gets into another person’s system.
- Safe handling of sharps
- Use gloves for procedures where there is risk of exposure to blood
- Use care in obtaining, transporting and processing specimens
- Perform good hand hygiene
- Mode of Transmission. Since microorganisms cannot travel on their own, they require a vehicle to carry them to other persons and places. See more detail in Section 4: Modes of Transmission.
- Mode of Entry. The path for the microorganism to get into a new host (the reverse of the portal of exit). The mode of entry refers to the method by which the pathogens enters the person.
- Susceptible Host. The future host is the person who is next exposed to the pathogen. The microorganism may spread to another person but does not develop into an infection if the person’s immune system can fight it off. They may however become a ‘carrier’ without symptoms, able to then be the next ‘mode of transmission’ to another ‘susceptible host’. Once the host is infected, he/she may become a reservoir for future transmission of the disease.
Susceptible hosts abound in health care settings, as those accessing the health care system often have compromised immune systems. This may be due to other illnesses processes, treatments or medications. This ineffective immune system leaves them vulnerable to infectious agents that may be in the health care environment.
Susceptible Hosts Control
- Children who are very young
- People who are very old
- People on inadequate diets
- People who are chronically ill
- People receiving medical therapy such as chemotherapy or high doses of steroids
- People who are already ill
- People with open wounds
- Separate high risk individuals from persons with known or potential infections
- Provide nutritional supplements to persons on inadequate diets
- Vaccinate against vaccine preventable diseases
- Maintain proper sanitation of air and environment
- Diagnose and treat underlying disease
Opportunities to break the chain of infection
Transmission may be interrupted when:
- the infectious agent is eliminated, inactivated or cannot exit the reservoir
- the portals of exit are contained through safe infection control practices
- the transmission between objects or people does not occur due to barriers and safe infection control practices
- the portals of entry are protected
- other persons receiving health care are not susceptible.
Diagram: Breaking the Chain of Infection
If the chain is not broken the infectious organism is able to go on to
develop disease in another person.
There are many opportunities to stop the spread of infection.
INFECTIOUS AGENT
SUSCEPTIBLE HOST RESERVOIRS
MEANS OF TRANSMISSION
PORTAL OF ENTRY
PORTAL OF EXIT
Bacteria Fungi Viruses Rickettsiae Protozoa
Rapid, accurate identification of organisms
Immunosuppression Diabetes Surgery Burns Cardio-pulmonary disease
Treatment of underlying diseases
Recognition of high-risk patients
Mucous membrane GI tract Respiratory tract Broken skin
Aseptic Technique
Catheter care
Wound care Direct contact Fomites Ingestion Airborne
Handwashing Sterilization Air flow control
Food handling
Isolation
INVOLVES
ALL HEALTH
PROFESSIONALS
YOU
People Equipment Water
Excretions Secretions Skin Droplets
Employee health
Environmental sanitation
Disinfection/ Sterilization
Proper attire
Handwashing
Control of excretions & secretions Trash & waste disposal
