Write what you know?

When I first started writing, one of the first rules I heard was "Write what you know." Well, I know about PT with kids, so did this rule mean I was supposed to write about PT with kids? That struck me as kind of boring. I was already doing it all day. Did I really want to write about it all night? My answer was 'no.'
The more I thought about it, I saw it as a challenge. Couldn't I take some of what I've learned and turn it into stories? I don't mean PT stuff like hamstring stretches or wheelchair prescriptions. I mean real lessons, like the dignity of the individual, how alike we all are underneath our bodies, how the disability of one person affects their family, and how different community members react to that person and their family. I was intrigued. And I started writing.
In the fiction novel I am writing, Sophie, the protagonist, develops polio. She goes from being an insider to being an outsider in her own society, an experience common for people with an acquired disability. She learns that her true value as a human being is not tied to her body. That's what the kids I work with have been teaching me for 30+ years.