What is antibiotic resistance, and how does it actually develop?
It is bacteria evolving, not people becoming resistant — a distinction that matters, because the common phrasing suggests your body stops responding to antibiotics, which is not what happens.
The mechanism. In any large bacterial population, random mutations mean some individuals are less affected by a given antibiotic. When the antibiotic is present, the susceptible bacteria die and the resistant ones survive and reproduce. The antibiotic does not cause resistance; it selects for it. This is natural selection operating over hours rather than millennia, because bacteria divide so quickly.
What makes it worse than ordinary evolution: bacteria can transfer resistance genes horizontally — passing them to unrelated bacteria, including different species, via plasmids. So resistance that emerges in one organism can spread to another without any inheritance at all. This is why resistance appears faster than the mutation rate alone would predict.
What drives it:
Unnecessary prescribing, particularly for viral infections, where the antibiotic does nothing except select for resistance in the bacteria you happen to be carrying.
Not completing a course — though this advice is now more nuanced than it was, with evidence that for some infections shorter courses are equally effective and may reduce selection pressure. Follow the course you were actually prescribed rather than a general rule.
Agricultural use, historically large-scale and for growth promotion rather than treatment.
Poor infection control, allowing resistant organisms to spread in hospitals.
Availability without prescription in some countries.
Why it matters so much. Antibiotics underpin far more than treating infections: surgery, chemotherapy, transplantation, intensive care and childbirth all depend on being able to prevent and treat infection. Losing them would make routine procedures dangerous again, which is why this is described as a threat to modern medicine rather than merely an inconvenience.
Why the pipeline is thin. Developing new antibiotics is commercially unattractive — a successful new drug is deliberately reserved and used sparingly, so it generates little revenue. This is a market failure, and various subscription-style funding models have been introduced to address it.
General information, not medical advice.