Influenza is caused by RNA viruses of the Orthomyxoviridae family that infect the respiratory tract. There are three main types of influenza viruses - A, B, and C. Influenza A is further divided into subtypes based on two surface proteins and can undergo antigenic drift or shift. Influenza spreads through respiratory droplets from coughing or sneezing. Symptoms include fever, cough, sore throat and fatigue. Vaccination and antiviral drugs can help prevent and treat influenza.