Just a thought...
It doesn't seem to me that you really even necessarily need to approach it in terms of the OCD (this is probably my biggest OCD issue too, btw). Many people without OCD don't really want people who are currently or very recently sick to come for a visit either(!) You mention that you don't know what kind of illness she was experiencing. Does she? What I'm thinking is, what if it was something totally not communicable, like, say, lady cramps? One might simply say they weren't feeling well, and not get specific with something like that. If it wasn't something of that nature, perhaps you could simply say that you'd feel better if she waited to visit till she's sure she's totally well again. To me, that's simply a matter of courtesy.
Good luck!