Common illnesses are illnesses that are widespread and affect many people. One of the most common illnesses is the cold, which is caused by a virus and is characterized by symptoms such as a runny nose, sore throat, and coughing. Other common illnesses include the flu, which is also caused by a virus and is characterized by fever, body aches, and fatigue; and the common cold, which is caused by a virus and is characterized by sneezing, congestion, and a sore throat.