Influenza — also known as the flu — is a contagious respiratory disease that can cause a mild to severe illness in patients. The normal course of illness runs from one to two weeks, on average. With that said, people have experienced a 24-hour variety of the illness. Many people think they are experiencing the 24-hour flu, actually have a pathogen that mimics flu symptoms in addition to diarrhea or nausea. So, what is the 24-hour flu?