Influenza virus is a highly contagious RNA virus primarily responsible for seasonal epidemics of respiratory illness, commonly known as the flu. Belonging to the Orthomyxoviridae family, it is classified into types A, B, C, and D, with types A and B causing most human disease. Transmission occurs via respiratory droplets, leading to symptoms such as fever, cough, sore throat, and body aches. Complications can include pneumonia, particularly in vulnerable populations. Annual vaccination is a key preventative measure.