“When we are sick with vomiting or diarrhea, we are also at increased risk of dehydration,” Dr. McAllister says. These are subtle signs of dehydration to keep in mind, according to doctors. Signs of dehydrationThese are some of the most common signs of dehydration, according to doctors. So when you don’t hydrate properly, your body goes through a thought process: What parts of the body need blood the most? Your brain is especially vulnerable to dehydration, Dr. McAllister says.