Flu, also known as influenza, is a communicable disease caused by the influenza virus. It spreads easily from person to person through respiratory droplets when an infected person talks, coughs, or sneezes. This means that if someone is infected with the flu, they can easily transmit the virus to others nearby. In contrast, diabetes mellitus, sickle cell anemia, and cancer are not communicable diseases; they are conditions that cannot be spread from one person to another. Understanding the difference between communicable and non-communicable diseases is crucial for public health and personal health management.