Antibiotics only work on bacterial infections.
People are notorious for treating something when they feel bad. Typically, they start to feel better 48-72 hours after starting antibiotics, and because they are feeling better they stop, or they simply forget.
What then happens is that the bacteria can multiply again, and much like the effect of a vaccination given to protect humans, the antibiotics act as a 'vaccination' - they are like a practice for the bacteria. They get stronger, and they become 'resistant' to the antibiotics. Not immune, but because they are 'resistant' they can survive long enough to multiply. Even if a few die off, the multiplication is 'exponenitial'. ie. instead of getting 2, then 4, then 6, then 8 bacteria, you get 2 then 4, then 8, then 16, then 32, then 64, then 128, then 256, then 512, then 1024, then 2048, 4056, etc.
So imagine a bacteria that multiplies every 30 minutes. Within 6 hours you have 4056 bacterial cells which originated from that 1 cell.
With resistant bacteria, the killing off is slowed, and they get killed slower than they multiply, so you can't get rid of them quick enough.
What happens then, is that you have to use less common antibiotics. But the side effects are stronger, and people still do what they did before. Those antibiotics become useless too.
There were real concerns some years ago, that the number of truly effective abx is reducing so far, that serious illnesses will become virtually untreatable.
Even high-grade antibiotics such as Gentamycin and Vancomycin now have bacteria which resist them.