We don’t have any official policy on this, but if you’re a bit under the weather and fear you might pass something on to others in the choir, then have a week practising at home. You won’t be prevented from singing in the concert if you’re ill just for a rehearsal — or even two if you do your homework!
The concert conductor’s decision is final if he does not want someone to perform in the concert due to rehearsal absences, though note this has only happened on very rare occasions!