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Home Topics Infectious Diseases Infections A-Z Healthcare Associated Infections (HCAI) General Information on Healthcare associated infections (HCAI)

General Information on Healthcare associated infections (HCAI)

A basic introduction to healthcare associated infection.

What is a healthcare associated infection?

Infections resulting from medical care or treatment in hospital (in- or out-patient), nursing homes, or even the patient's own home.   

Previously known as 'hospital acquired infection' or 'nosocomial infection' the current term reflects the fact that a great deal of healthcare is now performed outside the hospital setting.

Healthcare associated infection (HCAI) can affect any part of the body, including the urinary system (urinary tract infection), the lungs (puenmonia or respiratory tract infection), the skin, surgical wounds (surgical site infection), the digestive (gastrointestinal) system and even the bloodstream (bacteraemia). 

What type of infections are associated with healthcare?

This chart shows a breakdown of healthcare associated infections by body system for the United Kingdom:

Pie Chart showing the percentage of total healthcare associated system infection 

Source: Summary of Preliminary Results of the Third Prevalence Survey of Healthcare Associated Infections in Acute Hospitals 2006, England, Hospital Infection Society and Infection Control Nurses Association, 27 February 2007.





The presence of bacteria in the blood. The term 'fungaemia' is used if the micro-organisms in the blood are fungi (e.g. yeasts) rather than bacteria

Bloodstream infection

The presence of micro-organisms in the blood with signs of infection. This can be 'primary' i.e. inocculated directly into the bloodstream e.g. via an IV line or 'secondary' spread to the bloodstream from an original focus somewhere in the body e.g. urinary tract, etc.

Surgical site infection

Micro-organisms enter the tissues of the wound, and cause infection (also sometimes called surgical wound infection)

Urinary tract infection

The presence of symptoms or signs attributable to micro-organisms that have invaded the urinary tract (the organs involved in the production and excretion of urine) e.g. pain and frequency of urination

Respiratory infection

The presence of symptoms or signs attributable to micro-organisms that have invaded the respiratory tract (the lungs, wind pipe, nose and throat) e.g. cough, sputum and sneezing. Often caused by a virus rather than bacteria

Skin and soft tissue infection

Micro-organisms enter the tissues of the skin and related body tissues, often at the site of damage such as sore, ulcer or cut to cause symptoms such as redness, swelling and/or pus

Gastrointestinal infection

Micro-organisms infect the gastro-intestinal tract (digestive system: stomach and intestines) to cause symptoms such as diarrhoea, vomiting or abdominal pain

Many are caused by micro-organisms ('germs') already present in or on the patient's own body. Normally harmless, these cause problems only when the body's defences are breached by surgery, or other medical procedures.

Infections may also be caused by micro-organisms originating from another patient either by direct contact or spread via the hospital environment for example MRSA. 

With treatment most patients recover from a HCAI without any problems. However, these infections can extend a patient's stay in hospital, and is severe cases can cause prolonged illness, disability or even death.

The chain of infection

This diagram represents how a patient can acquire an infection from somebody else.  The links of the chain show how organisms are transferred and where healthcare staff can put prevention strategies in place to break the chain and stop infection spreading.

diagram showing chain of infection

Infectious agent Any disease causing micro-organism (pathogen):
e.g. bacterial, viral, fungal, parasitic, prion.
Reservoir Where a micro-organism normally lives and reproduces:
e.g. humans, animals, water, food.
Portal of exit The route of escape of the pathogen from the reservoir:
e.g. faeces, urine, wound discharge, mucus, blood, vomit.
Mode of transmission

The way the pathogen gets from the reservoir to the new host:

  • Contact
    • direct contact (actual contact with an infected person)
    • indirect contact (contact with contaminated surfaces touched by the infected person, or where droplets of body fluid have landed; Spread on unwashed hands) 
  • Airborne - "aerosols" tiny infected particles from an infected person released when they cough or sneeze which can be breathed in
  • Consuming contaminated food/water or swallowing of micro-organisms carried on the hands 
  • Blood exposures

Generally outside the healthcare setting:

  • Vector-borne (parasite bites), or sexual contact.
Portal of entry Inhaling, ingestion, breaks in the protective skin barrier (e.g. surgery, intavenous lines, injury), mucous membranes (mouth, eyes, nose).
Susceptible host

A person who can get sick when they are exposed to a disease causing micro-organism (pathogen).

Who gets HCAI?

Although anyone can get a HCAI some people are more susceptible to acquiring an infection.
There are many factors that contribute to this:

  • Illnesses, such as cancer, diabetes and heart disease, can make patients more vulnerable to infection and their immune system less able to fight it
  • Medical treatments for example, chemotherapy which suppress the immune system.
  • Medical interventions and devices for example surgery, artificial ventilators, and intravenous lines provide opportunities for micro-organisms to enter the body directly
  • Antibiotics harm the body's normal gut flora ("friendly" micro-organisms that live in the digestive tract and perform a number of useful functions). This can enable other micro-organisms, such as Clostridium difficile, to take hold and cause problems. This is especially a problem in older people.  

Long hospital stays increase the opportunities for a patient to acquire an infection. Hospitals are more "risky" places than the community outside: 

  • The widespread use of antibiotics can lead to micro-organisms being present which are more antibiotic resistant (by selection of the resistant strains, which are left over when the antibiotics kill the sensitive ones)
  • Many patients are cared for together - provides an opportunity for micro-organisms to spread between them.  

Where do micro-organisms causing HCAI come from?

The patient's own "flora"

Most infections, such as surgical wound infections and urinary tract infections, are caused by bacteria normally carried harmlessly on a patient's own skin (for example, Staphylococcus aureus) or intestine (for example, Enterococcus and E. coli). 

The environment

Some infections affecting patients with severely weakened immune systems, or those in intensive care can come from micro-organisms present in the environment - in or outside hospital (for example, Acinetobacter, Aspergillus and Pseudomonas).  These bacteria cause lung infections in particular.

Other people with an infection

Antibiotic-resistant micro-organisms (for example, MRSA, ESBL producers) are likely to have come from another infected or colonised patient, either via the hands of a healthcare worker, or through a contaminated hospital environment where an infected patient has been cared for. These can cause a range of infections.

Clostridium difficile, a cause of diarrhoea in a patient on antibiotics, can either be carried in the intestine or acquired from another patient with an infection. C. difficile makes spores which can survive for a long time outside the body allowing it to spread more easily. They are tough and resistant and not killed by, for example alcohol hand gels (so they need to be washed away with soap and water or destroyed by heat or powerful chemicals).

What is colonisation?

When someone is exposed to a micro-organism such as MRSA or C. difficile, they can become colonised. This means that the organism takes up residence harmlessly (for example on the skin, in the nose or in the bowel) but does not cause an infection. This colonisation may:

  • continue harmlessly indefinitely
  • clear spontaneously
  • develop into an infection.

Sometimes it is possible to use medical treatments to help the colonisation clear faster, e.g. before an operation.

What are antibiotic resistant infections?

Some HCAI are caused by antibiotic-resistant micro-organisms, such as MRSA (meticillin-resistant Staphylococcus aureus) and ESBL- (extended- spectrum beta-lactamase) producing organisms.  These do not usually cause infections that are different from those caused by non-antibiotic resistant micro-organisms.  The main difference is that they are resistant to the antibiotics most commonly used to treat these infections, which makes treatment more difficult. Generally they do remain sensitive to the more powerful antibiotics available in hospitals. 

For further information on antibiotic resistant infections.

Can HCAI be prevented?

Yes, some, but not all, healthcare associated infection can be prevented. In the past, infection was considered an inevitable consequence of medical and surgical treatment. It is now increasingly recognised that by a combination of good hygienic practice, careful use of antibiotics and improved techniques and devices, rates of infection can be lowered significantly.

What simple measures can help prevent HCAI?

  • Preventing spread between patients by cleaning hands either with soap and water or in some cases alcohol hand gel
  • Using "personal protective equipment", where necessary, for example, disposable gloves and aprons to prevent contamination of clothing and skin
  • Ensuring that, through regular cleaning, micro-organisms do not build up in the hospital environment
  • Isolating patients known to be colonised with a resistant micro-organism to reduce risk of spread
  • Careful use of antibiotics to minimise the spread of resistant strains and reduce the risk of patients contracting C. difficile infection.

Is there anything I can do about antibiotic resistance?

Yes, there is a lot you can do to help prevent the spread of antibiotic resistant micro-organisms.

  • Do not ask your doctor for antibiotics for illnesses like coughs, colds, or flu. These are caused by viruses, not bacteria, therefore antibiotics will not help, but may cause other bacteria in your body to become resistant. 
  • Expect to be given antibiotics only when they are really necessary, for a severe infection
  • Antibiotic resistance is most likely to develop when bacteria are exposed to antibiotics at levels too low to kill them outright. So if you are prescribed an antibiotic:
    • Take it exactly as prescribed and make sure you do not miss out any doses
    • Do not save any of your antibiotic treatment for the next time you get ill (different antibiotics are used for different infections, so they may not work anyway)
    • Never take out-of-date antibiotics. These lose their potency over time
    • Do not take antibiotics prescribed for someone else, as they may not be suitable for your infection and could even be dangerous.

What is the role of the Health Protection Agency (HPA) in HCAI and AMR?

The Health Protection Agency is an independent organisation, set up by the government to protect and public from the health effects of infectious diseases and other hazards to health. The Agency identifies, researches and responds to these threats. It gives advice to the public, provides data and information to government, and advises people working in healthcare.

The HPA monitors antibiotic resistance across England, Wales and Northern Ireland (surveillance in Scotland is undertaken by Health Protection Scotland, the Scottish counterpart of the HPA).

In England it is mandatory for health trusts to report all cases of blood stream infection caused by meticillin-resistant Staphylococcus aureus (MRSA) and glycopeptide-resistant Enterococci (GRE) and infections with C. difficile (CDI) to the HPA. Other infections (which make up the majority) are reported on a  voluntary basis. Hospital microbiology laboratories also end off resistant micro-organisms that they are concerned about to the HPA so they can be studied in more detail. The results of the surveillance are published regularly on the HPA website so that healthcare professionals can be made aware of changes in rates of infection and resistance. Additionally the HPA, in collaboration with the Department of Health, runs the Resistance Alert System, which tells microbiologists in the NHS about new and emerging resistance problems and how far they have spread.

Another HPA scheme is the Surgical Site Infection Surveillance Scheme, which helps hospitals monitor their own rates of post-surgical infection and compare themselves with similar organisations.

For more information

Antibiotic Resistance Prevention and Control (ARPAC) -

Alliance for the Prudent Use of Antibiotics (APUA) -

Advisory committee on antimicrobial infection and healthcare associated infection -

Centers for disease control and prevention (CDC) -

Bug investigator -

Department of Health -

Hospital infection society -

National Patient Safety Agency (NPSA) -

National Institute for Health and Clinical Excellence (NICE) -

E-bug - Antibiotic use and medicine and Hand hygiene

If you are concerned about your health, contact your GP or NHS Direct on 0845 4647, or visit