Influenza viruses are a group of viruses that cause the flu, a contagious respiratory illness. There are four main types: A, B, C, and D. Types A and B are responsible for seasonal flu epidemics, while type C usually causes milder respiratory symptoms. Type D primarily affects cattle and is not known to infect humans. These viruses spread through respiratory droplets when an infected person coughs or sneezes. Symptoms of the flu include fever, cough, sore throat, body aches, and fatigue. Vaccination is the most effective way to prevent influenza and reduce its spread in the community.