Sick-day reason can refer to any number of illnesses or medical conditions that require an individual to take a day off from work. In this case, the crossword solution for the term Sick-Day Reason is 'FLU', which is an infectious disease caused by the influenza virus. Symptoms of the FLU include fever, chills, body aches, and fatigue. It is highly contagious and can be spread through contact with an infected person or through airborne droplets.