Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. It can lead to symptoms like fever, cough, sore throat, body aches, and fatigue. The flu spreads easily from person to person, especially during the colder months when people are more likely to be indoors. Vaccination is one of the best ways to prevent the flu. The flu vaccine is updated each year to protect against the most common strains of the virus. While most people recover within a week or two, the flu can be serious, especially for young children, the elderly, and those with weakened immune systems.