When we get sick, our immune system mounts a response that fights back; our body then has WBCs and antibodies, which give us immunity for many years.
If we encounter the same pathogen again, then our immune system kills it quickly. But this protection doesn’t last forever. Why not?
The genes (DNA) inside the pathogens do not stay the same. In every new generation of bacteria there are random copying mistakes -> mutations.
Most mutations are “silent”. They don’t help, or hurt, the pathogen.
Some mutations are bad for the pathogen: they make it easier for us to kill them.
Some mistakes are good for the pathogen (makes it harder for us to kill them.) In this case, they are now more virulent. That’s good for the pathogen – but bad for us.
“The evolution of antibiotic resistance occurs through natural selection. Imagine a population of bacteria infecting a patient in a hospital. The patient is treated with an antibiotic. The drug kills most of the bacteria but there are a few individual bacteria that happen to carry a gene that allows them to survive the onslaught of antibiotic. These survivors reproduce, passing on the gene for resistance to their offspring, and soon the patient is populated by an antibiotic resistant infection — one that not only affects the original patient but that can also be passed on to other patients in the hospital.”
– University of California Museum of Paleontology’s Understanding Evolution (http://evolution.berkeley.edu).