The term "flu" refers to a contagious respiratory illness caused by influenza viruses. It is characterized by symptoms such as fever, cough, sore throat, runny or stuffy nose, muscle aches, and fatigue. Flu can range in severity from mild to severe, and it can lead to complications such as pneumonia, especially in high-risk individuals like young children, older adults, and those with underlying health conditions. The flu is typically spread through respiratory droplets when an infected person coughs or sneezes, or by touching a surface contaminated with the virus and then touching their mouth, nose, or eyes.