Question: What are the complications of Viral Gastroenteritis ?
Answer: Dehydration is the most common complication of viral gastroenteritis. When someone does not drink enough fluids to replace those that are lost through vomiting and diarrhea, dehydration can result. When dehydrated, the body does not have enough fluids to keep the proper balance of important salts or minerals, known as electrolytes. Infants, young children, older adults, and people with weak immune systems have the greatest risk of becoming dehydrated. The signs of dehydration in adults are - excessive thirst - infrequent urination - dark-colored urine - dry skin - lethargy, dizziness, or faintness Signs of dehydration in babies and young children are - dry mouth and tongue - lack of tears when crying - no wet diapers for 3 hours or more - high fever - unusually cranky or drowsy behavior - sunken eyes, cheeks, or soft spot in the skull Also, when people are dehydrated, their skin does not flatten back to normal right away after being gently pinched and released. People should talk with a health care provider if they have - blood in their stool, which may indicate a bacterial infection - symptoms that are severe or last more than a few days - symptoms of dehydration Severe dehydration may require intravenous fluids and hospitalization. Untreated severe dehydration can cause serious health problems such as organ damage, shock, or comaa sleeplike state in which a person is not conscious.
Source: NIDDK

Question: What causes Viral Gastroenteritis ?
Answer: Four types of viruses cause most cases of viral gastroenteritis. Rotavirus Rotavirus is the leading cause of gastroenteritis among infants and young children. Rotavirus infections are most common in infants 3 to 15 months old. Symptoms usually appear 1 to 3 days after exposure. Rotavirus typically causes vomiting and watery diarrhea for 3 to 7 days, along with fever and abdominal pain. Rotavirus can also infect adults who are in close contact with infected children, but the symptoms in adults are milder. Caliciviruses Caliciviruses cause infection in people of all ages. Norovirus is the most common calicivirus and the most common cause of viral gastroenteritis in adults. Norovirus is usually responsible for epidemics of viral gastroenteritis. Norovirus outbreaks occur all year but are more frequent from October to April. People infected with norovirus typically experience nausea, vomiting, diarrhea, abdominal cramps, fatigue, headache, and muscle aches. The symptoms usually appear 1 to 2 days after exposure to the virus and last for 1 to 3 days. Adenovirus Adenovirus mainly infects children younger than 2 years old. Of the 49 types of adenoviruses, one strain affects the gastrointestinal tract, causing vomiting and diarrhea. Symptoms typically appear 8 to 10 days after exposure and last 5 to 12 days. Adenovirus infections occur year-round. Astrovirus Astrovirus primarily infects infants and young children, but adults may also be infected. This virus causes vomiting and watery diarrhea. Symptoms usually appear 3 to 4 days after exposure and last 2 to 7 days. The symptoms are milder than the symptoms of norovirus or rotavirus infections. Infections occur year-round, but the virus is most active during the winter months. Viral gastroenteritis is often mistakenly called stomach flu, but it is not caused by the influenza virus. Some forms of gastroenteritis are caused by bacteria or parasites rather than viruses. More information about bacterial infections is provided in the NIDDK health topic, Foodborne Illnesses fact sheet from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).
Source: NIDDK