I'll be really blunt and this is a warning that you probably won't like what I have to say.

I work at a place that has tons of people calling in sick at any given time. If you work at my place and I tell you I really need you there and you insist on being out (and you aren't the person in the hospital), you will get a thanks and that's it from me. I'm not wishing you well--who has time to? I have to find someone to work in your place.

And if you call and tell me you're not coming--I'm cutting the conversation off after "thank you." Again, I have to find someone to work in your place.