Most of the illnesses that keep young children at home are viral. Antibiotics act on bacteria and have no mechanism by which they could affect a virus.
The two kinds of organism differ fundamentally
Bacteria are cells with their own machinery for building proteins and cell walls. Antibiotics work by disrupting parts of that machinery that human cells do not share.
Viruses have no such machinery. They enter a host cell and use its equipment to reproduce, so there is no separate target for an antibiotic to act on.
This is why the ineffectiveness is absolute rather than a matter of strength. A stronger antibiotic does nothing additional to a virus.
Symptoms do not distinguish them reliably
Fever, sore throat, cough and coloured nasal discharge occur with both. The colour of mucus in particular is a poor indicator, since it reflects the immune response rather than the organism.
The overlap is why clinicians frequently cannot tell at a single consultation. Duration, pattern and specific findings on examination carry more weight than any individual symptom.
Rapid tests exist for some organisms and are used selectively. For most childhood respiratory illness, testing every case would change management in very few of them.
What the immune response is actually doing
Fever is part of the response rather than the illness itself. Raised temperature alters conditions in ways that disadvantage many pathogens and speeds parts of the immune reaction.
Mucus production and cough are clearance mechanisms. They are unpleasant and they are also the means by which the airway removes debris and organisms.
Recovery from a viral illness therefore depends on the immune system completing its work, which takes a characteristic length of time that treatment does not shorten.
Unnecessary use carries real costs
Antibiotic exposure selects for resistant bacteria, both in the individual and in the wider population. Resistance accumulates and does not reverse easily.
There are individual costs too, including disruption of gut bacteria, diarrhoea and allergic reactions. These are not trivial in a young child.
Prescribing decisions balance these against the risk of missing a bacterial infection, which is why guidance often includes delayed prescriptions and defined review points.
When bacterial infection is genuinely suspected
Certain patterns raise suspicion: symptoms that worsen after initially improving, localised findings on examination, or illness continuing well beyond the expected course.
Some conditions are treated on suspicion because the consequences of delay are serious. That judgement depends on examination findings rather than symptom description.
A child who is unusually drowsy, breathing with difficulty, not passing urine, or has a rash that does not fade under pressure needs urgent medical assessment regardless of what caused it.