Influenza is a viral infection that attacks our respiratory system to our nose, throat and lungs. Influenza, commonly called the flu, is not the same as the stomach “flu” viruses that cause diarrhea and vomiting. The influenza virus, conversationally called the flu, is an illness that’s highly contagious and causes a number of different symptoms. More severe than the common cold, flu symptoms can cause serious complications like pneumonia and other infections in some people; especially those with a weakened immune system.