What is a nursing diagnosis for a patient with acute gastroenteritis and severe dehydration?

A/N/V/dizziness. If I understand question correctly, the nursing diagnosis should mainly address the actual signs and symptoms. So, severe dehydration should not be nursing diagnosis since technically you may call volume depletion by mistake a dehydration. But would be appropriate to say dizziness, weakness, low blood pressure as these are not disputable and not amenable to error.
Virus. Not sure what you mean by nursing diagnosis but most common causes of acute gastroenteritis are usually a virus. Some bacteria such as salmonella and ecoli can lso be a cause but a virus is more common.