A viral illness is an infection caused by a virus, which can be spread from person to person. The most common viral illness is the FLU, which is caused by the influenza virus. The FLU is usually characterized by fever, chills, body aches, and fatigue. It can also cause more serious complications such as pneumonia and bronchitis. Theflu is a more severe form of the FLU, which is caused by a different strain of the influenza virus. It is more contagious than the regular FLU and can cause more severe symptoms.