common illness

What Is A Common Illness? 1 : an unhealthy condition of the body or mind Germs can cause illness. 2 : a specific sickness or disease Colds are a common illness. illness. noun. What is the most common illness? Chest Cold (Acute Bronchitis) Cough, mucus. Common Cold. Sneezing, runny or stuffy nose, sore throat, cough.