They only kill one kind of thing
Why Don't Antibiotics Work on a Cold?
An antibiotic attacks a part that bacteria have and viruses do not, which is exactly why it does nothing for a cold.
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Step 1 of 5
A bacterium is a living cell.
It eats, builds and divides on its own, and to do that it needs a wall, machinery for making proteins and a way of copying itself. Each of those is a target.
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Step 2 of 5
A virus is not.
A virus is a set of instructions in a coat. It cannot eat or divide and does nothing at all until it gets inside one of your cells and uses your machinery.
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Step 3 of 5
So there is nothing for the antibiotic to hit.
Attacking a bacterial cell wall is useless against something with no wall, and attacking bacterial protein machinery is useless against something using yours.
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Step 4 of 5
Taking them anyway does harm.
Antibiotics kill the harmless bacteria you live with as well, and every course gives the survivors a chance to be the resistant ones that multiply next.
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Step 5 of 5
Resistance is why finishing a course matters.
Stopping early leaves behind exactly the hardiest ones. Resistant infections are already a serious problem, and new antibiotics are found far more slowly than resistance appears.
The first antibiotic came from a mould that had blown into a laboratory and killed the bacteria growing around it, and it took a decade more to make it usable.
The short version
Antibiotics attack structures only bacteria have, so they do nothing against a virus — and unnecessary courses breed resistant bacteria.
Try it yourself
Read the box next time. It will say what the medicine is for, and a cold will not be on it.