Species5618 wrote:
God changes them to have a fighting chance.
On a more serious note: Evolution. Bacteria reproduce much faster than humans or animals, so they evolve more rapidly. Random mutations may make specific bacteria resistent to antibiotics. If this happens incidentally, it's not really an issue, but if antibiotics are widely used, all strains that didn't develop this mutation will die out, while the strains that became resistent continue to thrive.
This, and bacteria can also transmit genetic material between themselves in a process called conjugation, making the spread of resistant DNA even faster.
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Here's to the crazy ones. The misfits. The rebels. The troublemakers. The round pegs in the square holes. The ones who see things differently.