Even when healthy, drink plenty of fluid every day. Drink more when the weather is hot or you are exercising.
Carefully monitor someone who is ill, especially an infant, child, or older adult. If you believe that dehydration is developing, consult a doctor before the person becomes moderately or severely dehydrated. Begin fluid replacement as soon as vomiting and diarrhea start -- DO NOT wait for signs of dehydration.
Always encourage the person to drink during an illness, and remember that a person's fluid needs are greater when that person has fever, vomiting, or diarrhea. The easiest signs to monitor are urine output (there should be frequent wet diapers or trips to the bathroom), saliva in the mouth, and tears when crying.
Barkin RM, Ward DG. Infectious diarrheal diseases and dehydration. In: Marx J, ed. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 6th ed. St Philadelphia, Pa: Mosby Elsevier; 2006:chap 171.
Landry GL. Heat injuries. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 688.