Addie, like most kids, liked to play outdoors, where she could be exposed to millions of bacteria. Addie was also a scab-picker, allowing some bacteria, namely staph (staphylococcus), to infect her.
While only a few bacteria infected her, they were constantly multiplying, so within hours there were millions of harmful bacteria in her body.
After going to the hospital, the doctors found that the staph had damaged her lungs so badly that she needed to be put on a lung bypass machine called ECMO. While ECMO could save a person's life, there was also a high probability that it would introduce new infections to the body.
Addie got a bacterial infection called Steno (Stenotrophomonas), which was immune to all but 5 or 6 antibiotics. However when doctors tried them one by one, they would work for a week or two, and then the Steno would flare up again. At the end, they declared the Steno pan-resistant, meaning that it was resistant to everything.
What had happened was some small part of the Steno was resistant to the antibiotics, due to genetic mutation, which was why it seemed like the antibiotics would work; the rest of the Steno had been killed off. However, after a week, the resistant Steno (blue) would have multiplied many times over.
After the cycle was complete with all of the antibiotics, the only Steno left were pan-resistant. This is an example of natural selection, because only the bacteria with traits that allow them to survive the antibiotics would live and reproduce.