My daughter is sick with a virus. Now she has diarrheaa what can we do for it?

Keep hydrated. The most important thing is to make sure your child stays hydrated. Give her small sips of water frequently. If she doesnt' want to eat solid food don't force it. She should be urinating at least every 8 hours and if she isn't please make an appt with your pediatrician to get her checked out.
Rehydrate. As dr. Webb points out, it is important to keep your daughter hydrated. It's important to rehydrate with electrolytes if possible. I don't necessarily recommend any particular brand, but products like pedialyte will do just fine. Don't use "sports drinks" unless that's all you have access to; if this is the case, dilute the sports drink 50/50 with water to make it isoelectric.