Transmission of infection in health care
Microorganisms that cause disease (bacteria, viruses, fungi including yeasts, protozoa, and prions) are transmitted and acquired in various ways. Some microorganisms need specific conditions to survive and multiply. For example, a certain temperature range, access to oxygen or protection from oxygen, and nutrients. When these conditions are met, they can multiply rapidly until killed/inactivated by the body’s immune system, by antimicrobial treatment such as antibiotics or, if in the environment (for example, a floor or other surface), by chemical treatment such as disinfectant.
In healthcare settings, pathogenic microorganisms are acquired mainly through contact, droplet or airborne transmission (Table A. Modes of transmission).
Contact is a common mode of transmission in health care, usually via touch or contact with blood or body substances.
Droplet and airborne transmission are not distinctly separate modes; transmission through the air is now understood to apply to a continuum of particle sizes, from large droplets to smaller particles that dry and remain airborne for hours. Transmission via aerosols (suspensions of liquid or solid in air) spans droplet and airborne modes.
Table A. Modes of transmission
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Mode
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Description and typical scenarios
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Examples
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Contact
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Microorganisms are transmitted by contact with contaminated body tissues or objects:
- direct transmission – transfer from one person to another (eg from patient to health worker via contact between blood, secretions or excretions and unprotected cut on the skin)
- indirect transmission (also called vehicle transmission) – transfer via a contaminated intermediate object (fomite) or person (eg via needles, equipment, contaminated surface or object, or hands after inadequate hand hygiene)
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Bacteria
- Clostridioides difficile (previously called Clostridium)
Methicillin-resistant Staphylococcus aureus
- Salmonella species
- Vancomycin-resistant Enterococci
- Vibrio cholerae (cholera) via contaminated water
- Staphylococcus and Streptococcus species causing impetigo
Viruses
- Hepatitis B virus via contaminated multidose vial or needles
- Monkeypox virus (direct or via fomites)
- Norovirus
- Rotavirus
- SARS-CoV-2 (COVID-19)
- Varicella-zoster virus
Parasites
- Sarcoptes scabiei (scabies)
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Droplet
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Large droplets (>5 micron) produced by an infected patient coughing/sneezing or through procedures (eg throat examination, suction, nebuliser use) can contact the mucous membranes (eyes, mouth and nose) of people within a 1-metre radius.
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Bacteria
- Bordetella pertussis (whooping cough)
- Neisseria meningitidis (meningococcal infection)
Viruses
- Adenoviruses
- Avian influenza viruses
- Influenza
- Monkeypox virus
- Mumps virus
- Norovirus
- Rubella virus
- Respiratory syncytial virus
- SARS-CoV-1
- SARS-CoV-2 (COVID-19)
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Airborne
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Aerosols containing particles <5 micron produced by an infected patient coughing, sneezing, talking or breathing, or through procedures (eg suction, nebuliser use) can remain suspended in the air for long periods and can be dispersed in air currents. Pathogens can be transmitted when susceptible people inhale contaminated air.
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Bacteria
- Mycobacterium tuberculosis
Viruses
- Rubeola virus (measles)
- SARS-CoV-1
- SARS-CoV-2 (COVID-19)
- Varicella-zoster virus
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Vector
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Microorganisms are introduced by another organism, such as an arthropod (eg mosquito, flea or tick) or rodent
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Unicellular parasite
Bacteria
- Borreliosis (relapsing fever)
Viruses
- Japanese encephalitis virus
- Ross River virus
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SARS-CoV-1: severe acute respiratory syndrome coronavirus 1; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2, the causal organism of coronavirus disease 2019 (COVID-19).
Some common microorganisms can be transmitted in multiple ways. For example, measles transmission is via airborne and droplet modes as well as direct contact with infected nasal or throat secretions and by indirect contact. Influenza and COVID-19 are spread by contact, droplet and airborne transmission.
Whether transmission of microorganisms causes clinical infection depends on the pathogenicity of the microorganism (ie its ability to cause disease) and the susceptibility of the person exposed. Other factors that affect risk of infection include the number of microorganisms transmitted (the dose) and contact with a cell type in which the microorganism can replicate (target cells).
Some people may become colonised by a potentially pathogenic microorganism but remain asymptomatic, and may become carriers who can transmit these microorganisms to others.
Resident microorganisms on skin, mucous membranes or in the gastrointestinal tract, which are normally harmless, can occasionally cause infection in the host individual (endogenous infection). This can occur due to:
- transfer from one part of the body to another (eg Escherichia coli from the gastrointestinal tract can cause urinary tract infection.)
- interruption of normal defences (eg a skin wound can become infected by resident skin flora such as Staphylococcus aureus.)
Infection prevention and control in office-based healthcare practices aims to prevent contact, droplet and airborne transmission of infections.
Resources
Australian Commission on Safety and Quality in Health Care resources supporting Australian Guidelines for the Prevention and Control of Infection in Healthcare
Basics of infection prevention and control. In: NHMRC. Australian guidelines for the prevention and control of infection in healthcare (2019)