Posted 5/9/2013 12:17 PM (GMT -6)
Your UC might be an autoimmune condition (no one really knows) but I think it's discussed that way in terms of your immune system overreacting to benign stimulation, kind of like you're having an allergic reaction to normal things in your bowel. (I'M NOT SAYING UC IS AN ALLERGY).
When you say your sister has "a flu," what do you mean? Does she have influenza (cough, fever, body aches) or does she have diarrhea? If she has diarrhea, you should probably stay away, because any bowel virus you catch is going to make your UC worse. If you catch influenza from her, you're going to be miserable enough just from the illness. The complications that UC might have for you is if you get a post-flu bacterial infection - treating with antibiotics can be complicated for someone with UC because antibiotics disrupt healthy bowel bacteria, which might trigger a UC flare for you.
Strep infections are not very contagious but if you get one you will have to work out the gut bacteria disruption from the antibiotics.