Understand the idea
Selection changes populations when heritable differences affect survival or reproduction in a particular environment. With antimicrobial exposure, resistant microbes can gain a relative advantage as susceptible competitors decline. Resistance traits can arise through genetic changes and spread between microbes; transmission then moves resistant organisms between hosts and settings. The patient's body does not become the resistant organism. [S09] This is population reasoning: a treatment does not require each bacterium to deliberately learn a new defense.
Worked example
In a deliberately simplified model, a population contains 990 susceptible and 10 resistant bacteria. After an exposure, 99 susceptible and 9 resistant organisms remain. Resistance rose from 1% to about 8.3%, even though the absolute resistant count fell. This illustrates selection, not a clinical prediction.
Where the idea stops
Real populations also involve mutation, gene transfer, ecological interactions, and changing resistance costs. The numerical example is a thought model, not prescribing guidance. [S09]
Connect it
Science and strategy: selection effects versus intentional adaptation.
Go deeper
Can the frequency of a trait rise while its absolute count falls?
Sources
US Centers for Disease Control and Prevention · Research reference · Date: Date not established
Explains resistance, selection pressure, and transmission without implying that human bodies become resistant.
Link checked 2026-10-17 · Public webpage accessed; no specialist peer review of this brief.