Short answer
Colonization means a microbe is present on or in the body without the evidence of host injury or response used to diagnose infection. Infection means the microbe has invaded or multiplied in a way that triggers a host reaction. A positive culture alone may not answer which one is present. 1 2
Colonization can precede infection, but detecting a microbe does not by itself establish infection.
On this page
At a glance
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| Question | Colonization | Infection |
|---|---|---|
| Microbe present | Yes | Yes |
| Host reaction or damage | Not detectable by the definition | Present or clinically significant in context |
| Example | Bacteria carried on skin without illness | Bacteria invading a wound and causing inflammation |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Colonization. Presence and growth of a microorganism on or in a host without detectable tissue damage or clinical host response in the stated context. 1 2
Infection. Invasion and multiplication of microorganisms with a host reaction, potentially with symptoms or tissue damage. 1 2
Key differences
CDC infection-control guidance distinguishes the two because colonized people can still transmit organisms while not themselves having an infection. Clinical assessment uses symptoms, site, testing and context; laboratory detection by itself may be ambiguous. 1 2
How to tell them apart
Ask whether there is evidence of invasion and a host response, not just a detected organism. A clinician interprets that evidence in context. Do not self-diagnose or assume antibiotics are needed from a culture result alone. 1 2
Where they overlap
The same species can colonize one person and cause infection in another, or change role in one person when circumstances change. Both states can matter for infection prevention. 1 2
Edge cases
Infection can be asymptomatic, so “no symptoms” alone is not a universal rule. Some microbial relationships fall between simple categories; different clinical definitions may use different thresholds. The comparison is about clinical use, not all microbiome ecology. 1 2
Why the distinction exists
The distinction prevents treatment decisions and infection-control measures from being based on microbial presence alone. 1 2
Common misconceptions
Colonization is not proof of an active infection, but it also does not guarantee zero transmission risk. 1 2
Examples
A nasal swab finds a bacterium without illness: this can be colonization. If that organism invades a surgical wound with signs of host response, it may be an infection. 1 2
Sources
Sources checked October 3, 2026.
- CDC — Infection Control Basics. Colonization and infection.
- CDC — Isolation Precautions Glossary. Colonization.