Over time, bacteria became resistant to antibiotics. Most people think that this is because the bacteria evolve new ways of dealing with them, but this is not usually true. Most antibiotics are derived from natural sources, such as other bacteria. Penicillin is produced by a mould. This mould has existed for approximately 200 million years. Bacteria live in the soil around this mould. Bacteria have therefore had 200 million years to develop resistance.

Obviously, this is limited to the bacteria that actually live near the mould. But bacteria are capable of transferring DNA between each other, allowing resistance genes to be transferred from bacteria that have to put up with the mould to other bacteria that haven't. Bacteria that produce antibiotics as a weapon will also have resistance genes, and are also able to transfer these to others. The net result is that a single resistance gene in the population can be transferred to the rest of bacteria on the planet in very little time.

Most resistance is due to this sort of thing. A resistance gene already exists and is inserted into other bacteria. This is much, much, much faster than evolution of an entirely new mechanism, and is the reason for bacteria now being immune to most antibiotics. Research is now being done into methods of interfering with the resistance mechanisms, on the grounds that there are unlikely to be genes that protect the resistance mechanisms.

Still, even if antibiotics do end up being completely useless it won't be the end of civilization. We survived for centuries without them, after all.