Symptoms. The flu is considered a respiratory system illness and often causes a variety of physical manifestations such as coughing, sneezing, elevated body temperature, nasal discharge, fatigue, sore throats, head and body aches, as well as fever. In certain cases, stricken individuals might also experience digestive issues including nausea, diarrhea, loss of appetite and vomiting. Though the ailment typically resolved within several days to two weeks in most otherwise healthy individuals, very young people, the elderly or those with compromised immune systems are at greater risk of developing dangerous, possibly life-threatening complications.