Why do viruses dehydrate you? i am not throwing up or have diarrhea...

They don't: They don't. You become dehydrated because you do not hydrate adequately in the face of fevers, nausea, diarrhea or loss of appetite caused by the virus.

Dehydraion: With virus infection, the person loses appetite and has poor oral intake: second nausea, vomiting and diarrhea which decrease the body volume, and third the fever causes fluid loss from skin.