My niece has just been diagnosed with hand, foot and mouth disease. what causes this?

Fever/spots/pain: Hfm is a viral infection. Some have fever, localized spots on their wrists & nothing else. Some have terrible mouth sores, spots on wrists/forearms/legs feet & around the bottom & have cramping & loose stools. Most have a mild case as kids and many go unlabled.Few have enough problems they become dehydrated. It is more a cosmetic issue than a threat.They are contagious until fever free.