Another problem is Improper Use of ABs:

Areas where ABs are available over-the-counter.

Or a poor family that can afford treating a single
member of the family, then as the bug spreads,
they share the prescription instead of completing
the proper regimen.

or taking them until they feel better, reasoning
they'll keep some in case they feel sick again.

Or self-medication with left-over ABs for an
unrelated malady, for which the AB isn't suitable.

Or taking them for the wrong reason altogether;
lots of people INSIST on their doctor prescribing
ABs for Flu, no matter what the doctor says.