Colds and flu (acute upper respiratory tract viral infections) are the most common diseases in humans. Adults have two to five common colds each year. It is a short, mild illness with early symptoms of headache, sneezing, chills and sore throat, and later symptoms of nasal discharge, cough and fatigue. Influenza (flu) is caused by infection with influenza A and B viruses; it is an unpleasant but usually self-limiting disease, with abrupt onset of fever, chills, non-productive cough, muscle pain, headache, sore throat and fatigue.
