Viruses vs. Bacteria
There are two kinds of infections that account for nearly all of pediatric infectious disease: infections by viruses and infections with bacteria.
Most infections are caused by viruses. Although there are some exceptions, infections caused by viruses are not treatable with antibiotics. Antibiotics work by killing bacteria. They have no effect on viruses.
Drug-resistant strains of bacteria
Being “resistant” to antibiotics is a characteristic of the bacteria in question -- not of the patient. There’s no such thing as a child for whom amoxicillin “doesn’t work,” although there certainly are children who have infections which amoxicillin does not treat in any way.
For instance, many of the bacteria which cause ear infections have become resistant to amoxicillin, mainly because of
inappropriate use of amoxicillin throughout the community. Even if your child has never taken amoxicillin, the bacteria which are part of our environment are more frequently resistant to amoxicillin than was the case 20 years ago, because of amoxicillin which has been used by others.
Another way of saying it is that the bacteria have evolved; the ones that could be killed by amoxicillin are no longer the ones that are circulating.
On the other hand, strep throat is an example of a relatively common problem where antibiotic resistance is not something we have to worry about. There’s no such thing as Group A streptococcus which is resistant to penicillin or amoxicillin – at least not yet.