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For this edition of MICKS, being the start of a new year, I felt it pertinent to go back to the fundamentals of infection prevention and control. This edition will reconsider the basic principles of infection prevention by examining the specific elements of the ‘chain of infection’ and how infection prevention and control strategies can ensure the safety of the people we care for, whether in hospitals, clinics or their homes.

What constitutes an infection?

In health, we exist in harmony with millions of microbes. These micro-organisms are in our environment, on our skin, mucous membrane and alimentary tract. Very few organisms are pathogenic (cause disease) and this eco system is mutually beneficial to both the organisms and human beings. Infection will only occur if there is an imbalance between the environment, the human host, and their normal bacteria (commensals).

The term infection is used to describe a situation where sufficient numbers of organisms reach a vulnerable site; they multiply and create an adverse reaction. If there is very little or no evidence of such adverse reaction this is termed colonisation.

The origin or source of the microbe which causes the colonisation/infection is classified into two groups - endogenous and exogenous. Endogenous infection, or self-infection, refers to the micro-organisms that exist harmlessly in one part of a person's body. However, they can become pathogenic if transferred to another, more vulnerable site. The normally non-pathogenic bacteria Staphylococcus epidermidis, for example, is a harmless skin bacteria but has a specific affinity for plastic devices such as intravenous cannulae and orthopaedic prosthesis. This bacteria, however, has been reported to cause a serious infection in the joints of patients who have undergone hip replacement with these plastic devices.

If the bacteria e.g. Staphylococcus aureus are not the patient's own, but those of a healthcare professional, or from contaminated equipment, this is termed exogenous infection (cross-infection). Chickenpox, measles, polio are classic examples of exogenous infection.

Some micro-organisms do not regularly cause disease in individuals with intact host defences. Pseudonomas aeroginosa, for example, is not always pathogenic, yet it can cause devastating diseases in many hospitalised immuno-compromised patients. These infections are referred to as opportunistic.

The Chain of Infection

Essentially there are three elements in the chain of infection:

  • Source
  • Means of transmission
  • A susceptible host

Principles of infection control are based on disrupting, or breaking, this chain of events. If a host becomes infected then, in turn, he/she becomes a source.


The source or the causative agent, may be endogenous or exogenous. Sources of infection can include other patients, staff, hospital equipment etc. The causative agent is a microbe, which may be a bacteria, virus, fungi, or a parasite. Disease does not necessarily follow invasion by a micro-organism; it depends on various factors. These factors include: the pathogenicity of the invading microbe; the number (dose) of organisms required to cause an infection; the virulence and specificity of the organism and.ability of the organism to survive after leaving the source.

(a) Pathogenicity
Pathogenicity of the micro-organism is a measure of its ability to induce disease. Pathogenicity of an organism varies according to three factors:
  • Virulence - a measure of the severity of the disease the organism can cause.
  • Invasiveness - an organism’s ability to invade tissue and overcome defences.
  • Toxicity - an organism’s capacity to damage tissue due to toxin production eg Clostridium difficile.

(b) Dose:
The number of organisms required to cause infection. This is referred to as the necessary infective dose. For example, large numbers of Staph. aureus e.g. 100,000 (105) can be applied to imtact skin, or injected, without causing a clinical infection; however, for salmonellosis, 106 organisms are required, but if gastric acid is deficient, then infection may be caused by a smaller number. Elderly patients receiving regular antacids may be more at risk of enteric infections because of the resulting high stomach pH i.e more alkaline.

(c) Specificity:
Micro-organisms are specific with regard to the range of hosts that they can affect e.g. Salmonella typhimunium only affects humans.

(d) Other microbial factors include the ability to produce enzymes to overcome defences, the production of new variants (strains); and drug resistance through the transfer of plasmids e.g. Methicillin Resistant Staphylococcus aureus.


Transmission is another link in the chain of infection and describes the movement of the organism from its source to another site.

An organism is only an infection risk if it is transmitted in some way to a susceptible host. Spread may occur through many different routes, although in some instances (Hepatitis C) the means of cross-infection remains uncertain.

An organism may have a single route of transmission, or it may be transmitted by two or more routes e.g. MRSA may be transmitted by contact and/or airborne routes. Salmonella species may be transmitted by contact, by indirect contact or may be vector-borne. Knowledge regarding the mode of transmission of a particular disease is important in controlling infection.

Classification of Methods of Spread

  • Contact spread - direct; indirect
  • Airborne - human; environment; aerosol
  • Vector- borne

  • Contact Spread
  • This may be direct or indirect

    • Direct
    • Cross-infection occurs when an individual has direct physical contact with the source of the infective organism. Methods of spread include contact with infected body fluids or with the hands of healthcare professionals. The latter situation is the most significant method of cross-infection, particularly for organisms such as MRSA. Bacteria that exist on our hands are either resident or transient flora. Resident bacteria are found deep in the dermis and epidermis of our skin. It is thought that they do not routinely cause cross-infection. Transient bacteria are organisms that are present on the superficial skin layer of our hands. These microbes are acquired during routine nursing care and are considered the bacteria most likely to cause infection. Fortunately, they can be easily removed by a thorough handwashing and drying regime.

    • Indirect
    • This is sometimes referred to as common vehicle spread and indicates that there has been indirect contact with the source which involved an intermediate object such as food, blood/blood products, antiseptic, fluids or equipment.

  • Airborne
  • Airborne transmission describes the spread of microbes that are disseminated in the air from their original source. There are three main potential sources of airborne spread.

    • * Human beings
    • Respiratory droplet e.g. Pulmonary Tuberculosis, Pneumonia, Measles Skins scales e.g. MRSA

    • * Environment
    • Dust, bacterial spores Water cooling towers and Showers e.g. legionella

    • * Equipment
    • Nebulizers e.g. gram negative Pseudomonas sp. Humidifiers e.g. bacilli

  • Vector- Borne

This refers to the mechanical transfer of micro-organisms in or on the body or appendage of the vector eg, Shigella sp. or Salmonella sp. can be transmitted by flies. Transmission can also occur when the agent goes through biological changes within the vector, as malaria parasites do within the mosquito. Following transmission to a susceptible host , the organism needs to enter the host and reach the vulnerable site where it is capable of causing an infection.


This is another important link in the chain of infection. The presence of an infectious organism does not necessarily result in pathology. Some people become carriers of potentially pathogenic organisms (HIV, Hepatitis B virus, MRSA). Although they remain symptom-free they can still transmit the organism to others. A variety of factors need to be overcome before infection occurs. A host's resistance to infection is dependent upon:

  • The general body defence mechanisms e.g. skin.
  • Innate (non-specific) immunity.
  • Acquired immunity, either passive or active.

  • General Body Defences
  • These include:
    • The physical defences such as intact skin and mucous membranes.
    • Desquamation (shedding) of surface cells together with adherent organisms.
    • Mechanical removal such as coughing and blinking and by ciliary action within the trachea.
    • Production of antimicrobial secretions e.g. lysozyme in sweat, tears, saliva and tissue fluid. Production of alkaline secretions (bile) also the acidity of sweat, gastric juice and vaginal secretions.
    • Immunoglobulins in blood serum, which are bactericidal.
    • Surface phagocytosis by macrophages, and epithelial cells.
    • Presence of commensal organisms in the upper respiratory tract, mouth colon, vagina, skin and stomach.
  • Innate Immunity
  • This is immunity an individual posseses as a result of genetic factors e.g. sex, age, race, familial factors and cellular factors such as phagocytosis and interferon. Innate immunity may also be influenced by a person's nutritional state, age and underlying disease.
  • Acquired Immunity
  • Active

    Acquired immunity is said to be active when a person develops their own antibodies. This can follow either an immunisation programme (Hepatitis B virus) or a sub-clinical, or clinical, infection (measles, chickenpox).


    Passive immunity occurs following the transfer of performed antibodies. This can occur transplacentally or following the administration of immunoglobin. Passive immunity is only short lived.


If the prevention of cross-infection is to be achieved, an infection control strategy must be integrated into nursing practice. There are numerous elements to the strategy, all of which are equal in importance and require both compliance and commitment. The strategy, however, should achieve three aims, each related to the elements of the chain of infection:

  • to control or remove the source (organism) e.g. decontamination of hospital equipment.
  • to block or interrupt the routes or mode of transfer of organisms, from potential sources to infected patients or staff, e.g. Isolation Procedures.
  • to enhance the patient’s resistance to infection and protect susceptible people. e.g. Hep B immunisation of staff, immunisation of children, protective isolation, good nutrition.

Compliance to an infection prevention strategy is achievable through implementation of pertinent infection control practice e.g. handwashing, decontamination procedures, effective linen management, isolation precautions etc. Commitment is generated through the provision of effective education, policies, management structure, and resources.


Safe infection control practice incorporates not only individual treatments and procedures but the environment also. The term ‘safe care environment’ is a broad term used in healthcare settings to signify a secure surrounding within which people are not harmed. In a hospital context, this refers to those processes which result in the reduction or destruction of contamination, the assessment of potential harm, and the implementation of care practices that minimise risk.

The initiation and maintenance of a safe environment is the responsibility of employees and employers.

Risk Assessment

Central to the development and maintenance of a safe care environment is risk assessment. Risk assessment is a modern management tool with particular relevance to infection prevention. Risk will not be totally eliminated by an effective risk management programme. However, good risk management procedures for infection prevention and control will allow staff to be aware of the potential risks and offer them the opportunity to address them before they result in cross-infection or harm to others.

Potential risks are present in most environments, and relate to the actual environment, the equipment, the people and the systems or practices implemented. Risk management in infection prevention should address each of these areas to ensure that a proactive approach is taken to infection control.

Risk management, in infection control, consists of a four phase cycle:

  • Risk identification i.e. What is the risk? What could go wrong? How could it happen? What would be the effect?
  • Risk analysis i.e. How often is it likely to happen? How much is it likely to cost? How severe would the effect be?
  • Risk control. i.e. How can it be eliminated? How can it be avoided? How can it be made less likely? How can it be made less costly?
  • Risk funding. i.e. money needs to be made available in order to address any residual risk and manage any existing risk. Therefore, investment in infection control is the key to the management of an effective healthcare service.

Thus, the components of a safe environment include:

  • Risk management procedures and policies
  • Routine hospital cleaning, disinfection and sterilization
  • Clinical waste management
  • Sterile services
  • Health and Safety regulations
  • Food safety and catering hygiene
  • Rigorous Care Practices

There are numerous care practices. However, some examples include:

  • Effective handwashing
  • Standardl Precautions
  • Effective management of linen and Clinical Waste
  • Systems of Isolation
  • Management of specific micro-organisms e.g. MRSA
  • Protective Clothing


This edition of MICKS has described the fundamental basis of infection prevention and control. The central elements of prevention in terms of infection and infectious disease is either:

  • to remove the source
  • block the mode of transmission and/or
  • increase the resistance of the host

All infection prevention and control practices relate to one or more of these fundamental elements.


University of Wales
Dr. John Gammon

Dr Gammon is recognised as an international authority on infection prevention and control. Currently, he is a Non -Executive Director of Carmarthenshire NHS Hospital Trust and Deputy Head of the School of Health Science, at Swansea University, Wales, UK. He has practiced as an infection control practitioner for many years and been instrumental in Wales in establishing infection control services. Furthermore he has lead on the establishment infection control courses, and national guidance on hospital and community infection prevention strategies. He has been central to the development of, evidence based, international guidance on patient isolation. His research interests include patient isolation, standard precautions and hand decontamination. He has published a number of research papers and continues to advise the Welsh Government on infection control practice and strategy. He acts as key advisor to a number of commercial companies involved in infection control. His focus of academic interest for the last few years has been the education and professional development of practitioners and specifically infection control practitioners. This has included e-learning course as well as Masters programmes in infection control.