In my situation when I was put on a verbal warning the very first time I stayed home sick I thought nothing of it. I wasn't planning on missing any more time and wasn't going to call in sick unless I really truly was.
When I get pneumonia and was put on written warning I became agitated and asked why. At the time, I worked overnights in a call center. From 2am-6am it was only myself and 3 other guys in the center. We each got a call about once every 30-45 mins during those hours so one person being out wasn't a big deal. My boss saw it a different way.
When I got the stomach flu about 2 months later my boss brought me down to speak to HR. He outlined my attendance history (missing work on 3 separate occasions) and the HR lady looked me in the eye and said to me in a voice eerily similar to the boss from Office Space:
"You know.... when I'm sick I still have to come to work."
Apparently even HR didn't care. No matter how sick I was and what a Dr. had to say about it, I was expected to be at work anyway.
I put in my 2 week notice 2 days later since I had a backlog of work at my consulting job that paid over twice as much.
My friend still works for the company and essentially it wasn't company policy to be so strict on attendance, just that my boss was a tiny man on a power trip. This same boss had even told another employee who had been there over 10 years and who missed work once a year on average that if he ever missed time from work without a Dr.'s note that he would be instantly fired.