I have a problem & need help finding a solution..

We have a bedroom in our home that is my brother's - but he's away for school, and only comes home for a couple days per quarter. Occassionally we have overnight guests who stayed there.

We never noticed there was a problem until my cousin (visiting from France!!) stayed there for a week. She got bitten all over her body with mosquitos like bites. Then for the last two times my brother came home, he was also bitten all over! But there was other guests who stayed in the room in between these two times but they didn't get bitten. It didn't happen to the other two bedrooms in the house. My mom even slept in my brother's room one day before he came home, thinking that whatever that bites him will bite her first, but nothing happened to my mom. Sure enough the next day my brother came home and was bitten all over!! My brother refused to sleep in his room during Spring break last week.. Seeing him sleeping on the sofa really breaks my heart

Tomorrow, my uncle and his wife will visit us from New York. They want to stay at my house, but I'm afraid they're going to get red bites all over their bodies!

Do you know of anything I can do to get rid of this problem fast?? The mattress was brandnew. We bought it about 1 1/2 year ago, but it only been slept on about 30 times total..

I want my brother to feel safe when he comes home again, and need to feel confident about having guests over. HELP?
__________________