However, in a voluntary society, each individual sets the standard by which they go. So when the employer interviews her, they say "Has an insured doctor ever examined you for disease? Did they conclude you were disease free? What is that doctor's name?" There's even more complex things like there could be cooks' insurance companies that will bond a cook for not having infectious disease, and the employer could just ask if they are bonded for it. And then when she is not bonded, they don't hire her. This put it on the employer, not on Mary.
And if an employer wants to take a chance and use her anyway, then they should be allowed, even if it would probably result in their certain death. This is the point of voluntaryism.
You make an excellent point. And in a voluntary society that had those options available, and the knowledge we have now, you'd be absolutely right. But the assumption at the time was that if you weren't
sick, you were healthy and fine. She was not, even though she was outwardly healthy.
The reason I don't trust you when you tell me I have a horribly infectious disease is not (necessarily) that you're not a doctor. It's because you haven't done a single test to establish that I am infectious. They performed a test, and combined with the fact that she had been leaving a trail of sick and dead employers, is sufficient evidence that she actually was infectious.
Let's update the case. We have a man, let's call him "Typhoid Marv." Here's the thing: He doesn't have typhoid. He has HIV. He doesn't know it, though, so he has a string of girlfriends who all come down with HIV or AIDS. They tell him to get tested, and he does. It (not surprisingly) comes back positive. Of course, he doesn't believe it, (he read on the internet that sometimes these tests give false positives) so he keeps having unprotected sex.
Is it the women's fault for not asking, or his for not telling?