My Dad's been really, really sick?!

My Dad's been sick for about a week and two days now. He's really tired and weak all the time. He's not coughing or stuffed nose, or anything of the sort. But weak, diarrhea, and tried is what he's feeling all the time.

I'm really concerned about it. I thought it was mono, but i dunno, he's had it before, but that was like 20 odd years ago. Can you guys give me a suggestion on what he has?