Topic 15.1 · Core
Antibiotics and Bacterial Infections
One relationship carries almost the whole of this page: antibiotics treat bacterial infections, and they do nothing at all to a virus. Students rarely get that fact wrong in isolation — where marks actually go missing is the explanation of why it's true, which needs a reason grounded in what bacteria and viruses actually are, not just a repeated rule.
An antibiotic is a drug used to treat bacterial infections. For the Core syllabus, three ideas need to stay separate in your head rather than blurring into a single vague statement: antibiotics are used against bacterial infections, antibiotics kill bacteria, and antibiotics do not affect viruses. Each one is doing different work in an exam answer, and a question can test any of the three on its own.
That gives you a simple rule to apply to any scenario a question describes: a bacterial infection is one an appropriate antibiotic may be able to treat, while a viral infection is one no antibiotic will touch, however severe the illness feels. A cold or influenza, for example, is caused by a virus, so giving antibiotics does not treat the virus itself — the illness has to run its course through the body’s own defences instead.
Why antibiotics do not work on viruses
The reason isn’t that antibiotics are simply “too weak” for viruses — it’s that they are not aimed at anything a virus has. Antibiotics act by interfering with structures or processes that bacteria specifically need, things like the machinery bacteria use to build their cell walls or to make proteins. Viruses are built completely differently: they are not cellular organisms, and they do not possess the same bacterial structures and processes an antibiotic is designed to disrupt. An antibiotic aimed at bacterial cell-wall construction has no equivalent target inside a virus, because a virus doesn’t have a bacterial cell wall — or any of the bacterial machinery the drug depends on — in the first place.
That’s a more precise explanation than saying antibiotics “can’t enter” a virus or a host cell — the safer and more accurate reasoning Cambridge wants is that a virus simply lacks the bacterial targets antibiotics act on, not that the drug fails to reach its target once inside the body.
Bacteria vs viruses, as far as antibiotics are concerned
| Bacteria | Viruses |
|---|---|
| Cellular organisms, with their own cell wall and internal machinery. | Non-cellular infectious particles, built without a bacterial cell wall or bacterial machinery. |
| Possess the structures and processes antibiotics are designed to target. | Do not possess those bacterial structures or processes at all. |
| A bacterial infection can potentially be treated with an appropriate antibiotic. | A viral infection is not affected by antibiotics, whatever dose is used. |
What a complete exam answer looks like
A question that asks why antibiotics won’t help with a viral illness isn’t looking for “because it’s a virus, not bacteria” on its own — that restates the question rather than answering it. It wants the causal step in between: antibiotics act on bacterial structures and processes, and a virus does not have those structures and processes, so there is nothing for the antibiotic to act on. Naming that missing target is what turns a fact you’ve memorised into an explanation an examiner can actually award.
It’s also worth being careful with the reverse direction. Not every bacterial infection needs treating with an antibiotic, and a strong answer never implies that antibiotics are a universal cure for anything caused by bacteria — only that bacterial infections are the category antibiotics can act against, which is a different and more careful claim. For the underlying difference between what a pathogen is and how an infection actually spreads, that groundwork sits in pathogens and disease transmission.
None of this guarantees an antibiotic will always work, though. Some bacteria don’t respond to a given antibiotic at all, and that idea — antibiotic resistance — is where the chapter goes next.