The influenza virus, commonly called "the flu", infects the respiratory tract of humans and animals. It is caused by RNA viruses in the Orthomyxoviridae family. There are three main types of influenza viruses - A, B, and C. Common symptoms of the flu include fever, cough, sore throat, runny nose, headache, muscle aches, and fatigue. Treatment focuses on vaccination with flu shots or nasal spray vaccines, increasing fluid intake, and using warm showers or compresses to relieve symptoms.