Colonisation, Infection and Disease

These terms are used precisely in clinical microbiology and are frequently confused. Distinguishing them correctly is the single most important interpretive skill in reading a microbiology result.

• The accidental, incidental presence of microorganisms in a specimen or on a surface, without true colonisation or infection of the host.
• Typically arises during specimen collection (skin flora entering a blood culture bottle) or environmental exposure, not from a genuine host-organism relationship.
• Clinically important because a contaminated specimen can generate a false-positive impression of infection if not recognised as such.

• Microorganisms are present at a body site and actively multiplying, but there is no tissue invasion and no clinical disease.
• The host mounts no significant symptomatic response.
• Colonisation is the normal state for the resident microbiota (previous subsection) and can also occur with organisms capable of causing disease under other circumstances (e.g. MRSA nasal colonisation, S. pneumoniae nasopharyngeal carriage), where the organism is present without producing illness in that host at that time.

• Microorganisms invade or multiply within host tissue, triggering a host response.
• Infection may or may not produce clinical symptoms: subclinical (asymptomatic) infection is common and is distinguished from colonisation by the presence of tissue invasion and/or a measurable host immune response, even without symptoms.

• Infection that produces tissue damage, physiological disturbance, or clinical symptoms.
• This is the narrowest of the terms covered here: infectious disease is a subset of infection, and infection is a subset of the broader category of organism presence that also includes colonisation.

• The host harbours and may transmit an organism without exhibiting obvious disease.
• Distinguished from simple colonisation by its emphasis on ongoing transmission risk to others, a carrier is a potential source of infection for other people even while remaining personally asymptomatic (classic example: Salmonella Typhi chronic carriage, historically illustrated by "Typhoid Mary").

• The organism persists within the host in a nonreplicating or minimally active state, without producing active symptomatic disease, but retains the capacity to reactivate later.
• Distinguished from carrier state by the absence of active organism shedding/transmission during the latent period in most cases (though this varies by organism).
• Classic examples: herpesviruses (varicella-zoster virus latency in dorsal root ganglia, with later reactivation as shingles), Mycobacterium tuberculosis (latent TB infection, with a lifetime reactivation risk), HIV (proviral latency within resting CD4 T cells).

• Infection occurring when host defences are impaired or normal anatomical/physiological barriers are disrupted, typically caused by an organism of low intrinsic virulence that would not cause disease in a host with intact defences.
• Directly connects back to the Opportunistic Organisms concept introduced in the Introduction to Microbiology subsection: the organism has not changed, the host's capacity to control it has.

• Correctly categorising a given clinical scenario using this hierarchy is the foundation for every later decision about whether to treat, whether a positive culture is clinically significant, and how to counsel a patient about transmission risk. This framework is applied repeatedly throughout Diagnostic Microbiology and System Based Clinical Microbiology.