Via WHO's Western Pacific Regional Office: Hand, Foot and Mouth Disease Information Sheet. Excerpt:
How serious is HFMD?
Most people with HFMD recover fully after the acute illness.
•HFMD is usually a mild disease, and nearly all patients recover in 7 to 10 days without medical treatment and complications are uncommon.
•Dehydration is the most common complication of HFMD infection caused by coxsackieviruses; it can occur if intake of liquids is limited due to painful sores in the mouth.•Rarely, patients develop "aseptic" or viral meningitis, in which the person has fever, headache, stiff neck, or back pain, and may need to be hospitalized for a few days.
•HFMD caused by EV71 has been associated with meningitis and encephalitis, and on occasion can cause severe complications, including neurological, cardiovascular and respiratory problems. Cases of fatal EV71 encephalitis have occurred during outbreaks.