Dumb answers above. The issue is not that she is sick. The issues is dependability. Yes, fire her. She blew her probationary period twice now. Bugs going around doesn't mean everyone will get it. If she can't keep herself healthy to do the job, and she already took off once during her probationary period, I'd fire her. She is not dependable. She can be the greatest employee of them all but if she isn't there when she should be, and she's only 3 weeks into the job, she won't be any help when she isn't there.
Certified Paralegal, with 25+ years' experience & with Employment law experience.