The major organ affected by the flu virus is the respiratory system, specifically the lungs. When the influenza virus infects an individual, it primarily targets the epithelial cells lining the respiratory tract, leading to inflammation and various symptoms associated with the flu, including coughing, sore throat, and difficulty breathing. The flu can cause pneumonia, a serious respiratory infection, particularly in vulnerable populations, such as the elderly or those with weakened immune systems. Effective prevention measures, such as vaccination, play a crucial role in reducing flu-related complications and protecting the respiratory system during flu season.