Final answer:
Intestinal flu, or gastroenteritis, is most commonly caused by a virus, such as rotavirus in children and norovirus in adults. Other potential causes include fungi and protozoa.
Step-by-step explanation:
The most common cause of intestinal flu, also known as gastroenteritis, is a virus. In children, rotavirus is often the culprit, while norovirus is more likely to cause the illness in adults. Other potential causes of gastroenteritis include fungi and protozoa.