What is an influenza and what disease is caused by it?

Influenza: Influenza is a viral infection that attacks your respiratory system — your nose, throat and lungs. Influenza, commonly called the flu, is not the same as stomach "flu" viruses that cause diarrhea and vomiting. http://www.mayoclinic.org/home/ovc-20248057

Known as "the flu": influenza is an infectious disease caused by the influenza virus (there isn't "an influenza"). Typical symptoms: high fever, runny nose, sore throat, muscle aches, headache, cough, fatigue. It spreads through the air from coughs/sneezes or touching contaminated surfaces. Yearly preventive immunizations are effective. Treatment: rest, fluids, fever/pain relievers. Antivirals may help if given early