Seasonal illness is a term used to describe illnesses that occur more frequently during certain times of the year. The most common seasonal illness is the FLU, which is caused by a virus and is characterized by symptoms such as fever, chills, body aches, and fatigue. The FLU is highly contagious and can be spread through contact with an infected person or through airborne droplets. Vaccines are available to help prevent the spread of the FLU, and it is important to get vaccinated each year to reduce the risk of infection.