What is going on?!

The past six weeks, EVERY Sunday, one of my children have come down with a bad headache and then thrown up. It first started with Elena, who gets migraines occasionally, so I didn't think too much of it. Then it moved to Joseph, and then Rebecca. It starts either late Saturday night or Sunday, and the longest it has lasted has not been more than 12 hours, until yesterday. I don't know if it has something to do with staying up a little later on the weekends, or what causes it, but I feel so bad for them. This is Becca's second week in a row. (She was fine Monday through Saturday) Elena had it two weeks, (Sunday night only both times) Joseph had it 1 week, (Sunday night only) Elena got it again (Sunday night only) and then Rebecca had it last week, (Sunday night only) and again this week, (only I'm thinking this might be the flu, because she keeps throwing up). I have scrubbed the house, cleaning every room, but then I started to wonder about our leaking roof. It has been leaking for a year and every time it rains or snows, or melts, we have to put a bucket on the staircase. Well it has gradually expanded and we now have to put a bowl on one step and a bucket on the next. I got looking at it and I think it is starting to mold. Could this make people sick? Why isn't anyone else sick if so? Any ideas? Elena says next week it will be Kimball, but I hate it when the small ones get sick, because they can't quite make it to the toilet, and it sure gets messy.