Dehydration is caused by not drinking enough fluid or by losing more fluid than you take in. Fluid is lost through sweat, tears, vomiting, urine and diarrhoea.
- Dehydration @ NHS Choices
Excerpt: Dehydration is usually caused by not drinking enough fluid to replace what we lose. The climate, the amount of physical exercise you are doing (particularly in hot weather) and your diet can contribute to dehydration. You can also become dehydrated as a result of an illness, such as persistent vomiting and diarrhoea, or sweating from a fever.