Why don't antibiotics work on colds?
Antibiotics attack machinery that bacteria have and viruses do not — and a cold is a virus, which has almost no machinery of its own to attack.
Simple intuition
The plain reason, in everyday words
Bacteria are living cells. They have an outer wall to build, machinery for making proteins, and their own way of copying their DNA — and every antibiotic works by jamming one of those. Penicillin, for instance, stops a bacterium building its wall, so it bursts as it tries to grow. Now look at a virus. A virus is essentially a set of instructions in a protein shell. It has no wall to build, no protein factory of its own, no metabolism to poison. It gets inside one of your cells and makes your cell do the work. So an antibiotic arrives looking for the machinery it was designed to sabotage and finds none of it — and worse, the machinery that is running the infection is your own, which the drug must leave alone. This is a mismatch of target, not a matter of dose or strength.
Antibiotics will help a bit, or speed things up slightly.
Against a virus they do nothing at all to the infection. There is no partial effect, because the structures they act on are not present.
Green mucus means a bacterial infection and therefore antibiotics.
The colour comes from an enzyme released by your own immune cells and appears in ordinary viral colds. It does not distinguish between causes.
Resistance is something your body develops.
The bacteria become resistant, not you. That is why resistance is a shared problem — the resistant organisms circulate, and someone who has never taken an antibiotic can acquire a resistant infection.
Taking antibiotics 'just in case' is harmless if they turn out not to be needed.
Each course disturbs the normal bacterial populations of the gut and skin, carries side effect risk including C. difficile infection, and adds selection pressure. The cost is real even when the benefit is zero.
It replaces a vague sense that antibiotics are general-purpose medicine with the specific reason they are not, which is the difference between accepting 'no antibiotics today' as a refusal and understanding it as a statement about what is wrong with you. It also introduces selective toxicity, the principle behind nearly every safe drug: find something the invader has that you do not. That is why antibiotics were comparatively easy to discover, antivirals are hard, and cancer drugs are hardest of all.
Who worked it out
Paul Ehrlich set out the goal of a 'magic bullet' — a compound toxic to a pathogen and harmless to the patient — and produced Salvarsan for syphilis in 1909, establishing selective toxicity as the organising idea.
What problem forced it
Alexander Fleming observed penicillin's effect in 1928, and Florey and Chain developed it into a usable drug in the early 1940s, transforming bacterial infection from a common cause of death into a treatable condition.
How it changed since
Resistance appeared almost immediately — Fleming warned of it in his 1945 Nobel lecture — and the discovery of new antibiotic classes slowed sharply after the 1980s, turning stewardship of the existing ones into the central concern.
How antibiotic resistance spreads between species
Resistance genes travel on mobile genetic elements, which is why pressure in agriculture shows up in hospitals.
Why antiviral drugs are so much harder to develop
The same reasoning in reverse: fewer targets that belong to the virus rather than to you.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.