Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin one to four (typically two) days after exposure to the virus and last for about two to eight days. Influenza, or “the flu,” is a contagious disease of the nose, throat, and lungs, caused by a virus. The disease can range from mild to severe. The elderly, very young children, pregnant women, and people with certain health conditions become infected more easily, and these infections tend to be the most severe. Learn about identifying flu symptoms, how the diagnosis works, common complications, and more. Getting vaccinated every year is the best way to avoid getting sick with the flu. The flu, common cold and COVID-19 have similar symptoms. The flu and COVID-19 can be severe, but colds rarely are. What is the flu (influenza)? The flu is an illness you get from the influenza virus. Influenza (Flu) in Arizona ADHS Home Public Health Preparedness Epidemiology & Disease Control Infectious Disease Services Highlighted Infectious Diseases for Arizona Influenza (Flu) in Arizona Menu Public Health tracks flu and RSV cases by MMWR weeks, which is a standard way across the United States of numbering weeks in a year to track disease. Weekly reports start on Sunday and end the following Saturday. All data are preliminary and may change as more reports are received. Flu, also called influenza, is an infection of the nose, throat and lungs, which are part of the respiratory system. The flu is caused by a virus. Influenza viruses are different from the stomach flu viruses that cause diarrhea and vomiting. Most people with the flu get better on their own. Flu is a contagious respiratory illness that infect the nose, throat, and sometimes the lungs. Learn about the current flu season and find resources related to flu surveillance and past seasons. Learn more about who is at higher risk of developing potentially serious flu complications. Millions of Americans Think They Have the Flu. Most of Them Are Wrong. Are you one of them? Every season, millions of Americans check the flu forecast, reach for a painkiller or cold medication and wait for the symptoms to pass. But somewhere along the way, something doesn't add up. Copyright © 2026 IQVIA™. All rights reserved. Where is flu active today? Get Cold & Flu Activity Report for Phoenix, AZ (85018). Check Cold & Flu conditions in your area.