Someone wrote the software for the equipment they use. I was recently at the hospital for surgery, and there were scant few pieces of equipment that didn't have some kind of programming in it.
Maybe a small percent of software is written for life-critical systems. That's because software is a huge market. But a growing percent of life-critical systems rely on software. How many adverse outcomes (even death) are you willing to accept caused by programming failures?
You know, a big part of the prosecution's evidence in the Casey Anthony trial turned out to be false; it was due to faulty software. She could have been imprisoned due to a software bug! http://en.wikipedia.org/wiki/Casey_Anthony#Evidence
Now consider that anybody can call themselves a software engineer without even picking up a book. Next time your life hangs on the proper functioning of some computer system, think about that.
In percentages, sure. In the raw number of people, I imagine the numbers are a lot closer than you imagine.
As for the remaining percentage of people, we may not deal with their physical health, but our work can impact their financial or family's well being fairly easily. Some of the code I have written has touched billions of financial transactions that have decided the future of whether people will be able to buy a house, or a car, or any other line of credit.
What % of doctors have an impact on the health of their patients? Almost all.
We are dealing with statistics here.