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Basic Concepts - An Overview

Infection prevention and control (IPC) is the application of microbiology in clinical practice to prevent the transmission of infection caused by bacteria, fungi, viruses and prions. Infection can acquire either from:

  1. An endogenous route where the source of microorganisms is from the patient’s microflora. For example, break in intestinal mucosal barriers caused by chemotherapy in cancer patients
  2. An exogenous route where the microorganisms are from other sources. For example, cross-infection due to contaminated hands of healthcare workers, items, equipment, and/or environmental sources, i.e. environmental surface, water, and air.

The transmission of infections and outbreaks in healthcare facilities occurs mainly via the exogenous route and has an adverse outcome for patients and healthcare workers. Therefore, understanding how cross-infections occur is essential to implement measures to prevent their spread.


The chain of infection refers to a process where microorganisms are transmitted from a source or a reservoir to a susceptible host. Fig. 1 summarises the chain of transmission.


The source or reservoir of microbes (bacteria, viruses, fungi etc.) can be humans (infected/colonised individuals, asymptomatic carriers), animals, or environmental sources (esp. frequently touch environmental surfaces), water, e.g. Legionella and Pseudomonas spp. and air, e.g. Aspergillosis from building work in immunosuppressed patients).

Fig. 1 Chain of infection




In healthcare, microorganisms are mainly transmitted through contact, droplets and inhalation. In addition, other modes of transmission also include ingestion (faecal-oral route) and inoculation (sharp injuries). However, it is important to remember that some microorganisms can be transmitted through multiple routes. Examples include MDROs and respiratory pathogens, e.g. Respiratory syncytial virus (RSV), influenza, SARS, etc.

Contact transmission is the most frequent transmission of healthcare-associated infections (HAIs). It can occur as direct contact when there is physical contact with a patient during medical examination, bathing, dressing changes, insertion and maintenance of invasive devices, etc. Therefore, performing hand hygiene as per WHO 5 Moments of Hand Hygiene, removing contaminated gloves following clinical activity, and disinfection hands following glove removal are the most effective means of preventing transmission through this route.

Transmission through indirect contact occurs when pathogens are transmitted through intermediate objects, i.e. via contaminated items, equipment, and/or the environment. Therefore, effective decontamination of medical items and equipment in healthcare settings and cleaning and disinfection of environmental surfaces (esp. high-frequency hand touch surfaces) is critical to prevent transmission through this route.

Examples of pathogens that can spread via contact include microorganisms causing diarrhoea (C. difficile, Norovirus, etc.), Scabies, herpes simplex, Ebola, infected and/or colonised patients with Multidrug resistant Organisms, etc.

Droplet transmission is the spread of an infectious agent caused by the dissemination of droplets. Droplets are primarily generated from an infected (source) person during coughing, sneezing and talking. Transmission occurs when droplets that contain microorganisms are propelled (usually < 1 m or ~ 3 feet) through the air and deposited on the conjunctivae, mouth, nasal, throat or pharynx mucosa of another person. Most of the volume (> 99%) comprises large droplets that travel short distances (< 1 m or ~ 3 feet).) and do not remain suspended in the air as these large particles will fall quickly to the ground in a few seconds due to gravitational force. Direct transmission via this route can be prevented by wearing facial protection (surgical mask and/or face shield). In healthcare facilities, sufficient space (at least 1 metre or more) should be provided between patients to prevent cross-infection and to carry out clinical activities. Thus, special air handling and ventilation are not required to prevent droplet transmission.

Examples of pathogens that spread via droplet transmission include influenza and other respiratory viruses, Neisseria meningitides (meningococcal meningitis), rubella virus (German measles), mumps, and Bordetella pertussis (whooping cough ).

Airborne transmission is the spread of infectious agents caused by the dissemination of droplet nuclei that remain infectious when suspended in air over long distances and time. It is recommended that patients requiring airborne isolation precautions are cared for in a negative pressure ventilation room (6–12 air changes/hour) to dilute and remove the infectious microorganisms safely. The ventilation rate should be 6-12 air changes per hour (e.g., equivalent to 40-80 L/s/patient for a 4x2x3 m3 room), and ideally 12 air changes per hour for new facilities, with a recommended negative pressure differential of ≥2.5Pa (0.01-inch water gauge) to ensure that air flows from the corridor into patient rooms. In addition, amongst other measures, the healthcare worker should wear a respirator (FFP3) before entering the room.

Examples of pathogens that spread via airborne transmission are Mycobacterium tuberculosis (tuberculosis), varicella-zoster virus (Herpes zoster/shingles), and rubeola virus (measles).

Types of Isolation

Source Isolation: The objective of source isolation is to isolate the infected/colonised patient in a single room with an en-suite toilet facility. The old term’ barrier/precaution nursing’ should be avoided. If a single room is unavailable, cohort patients with similar symptoms and diagnosis and/or infected or colonised with the same microorganism. Cohorting should only be done on the advice of the local IPC team/Public Health Agency, and a dedicated staff team should care for these patients. This can only be implemented if sufficient staff are available.

Airborne infection isolation rooms are at negative pressure. Where possible, this room should have an anteroom. They are used if the infection is transmitted by airborne route. These rooms are negative pressure, which means air flows from the corridor to inside the patient room, and 6 to 12 air exchanges per hour are required. The exhausted air goes outside the building, away from places where people walk or congregate and any air intake openings. The door is kept closed when not required for entry and exit. If an airborne infection isolation room (AIIR) is not available, the patient(s) should be placed in a well-ventilated area with doors closed.

Protective Isolation: Many infections acquired by immunosuppressed patients are endogenous infections, i.e. infections caused by microorganisms already present in the body but previously inapparent or dormant. However, due to their increased susceptibility, the transmission of infection from other patients, health workers, or the environment (e.g. aspergillosis) can be a risk, and therefore additional precautions are required.

Patients requiring protective isolation should be nursed in a single room at a positive pressure supplied with good quality filtered air maintained at a positive pressure with respect to surrounding areas. The recommended air change should be ≥10 per hour. Where possible, this room should have an anteroom. The room should have en-suite toilet facilities, and the doors(s) should always be closed. For maximum effect, only one of the doors in the anteroom should be open at any time when entering or leaving the room. It is important to note that in some cases, if an immunocompromised patient has a concurrent communicable disease, source isolation may be required, and positive pressure ventilation may be inappropriate. These patients should be discussed with the IPC Team/Microbiologist/Clinician.


Infection occurs in a susceptible host, especially those without adequate immunity through past infection or immunisation. Preterm neonates, the elderly population, individuals with organ or bone marrow transplants, patients on immunosuppressive therapy, chronic disease, and pregnant mothers are more susceptible to infection.

Depending on the type of microorganisms and susceptibility of the host, it is important to note that exposure to microorganisms does not always lead to infection. The possible outcomes of an exposed individual to microorganisms are summarised in Fig. 2. 

Fig. 2 Possible outcomes if an individual is exposed to an infectious agent.

Source: Damani N. Manual of Infection Prevention and Control (4 ed). Oxford: Oxford University Press, 2018.