Flu, a common name for influenza, is an infectious disease caused by RNA viruses (influenza viruses). Runny nose, sore throat, cough, chills, fever, muscle pain and weakness, severe headache, etc. are some of the symptoms. It may cause nausea and vomiting. Influenza is more severe than the common cold. It is typically transmitted through coughs or sneezes, by direct contact with bird droppings or nasal secretions or through contact with contaminated surfaces.