What defines a communicable disease?

A communicable disease is defined as one that can be transmitted from one human to another. This encompasses illnesses caused by infectious agents such as bacteria, viruses, fungi, or parasites that can spread through direct contact, airborne pathways, or various other modes of transmission. The ability for these diseases to be passed among individuals is what categorizes them as "communicable."

Other options describe characteristics that do not capture the essential nature of communicable diseases. For example, while some communicable diseases can be treated with antibiotics, this is not a defining characteristic of them since not all communicable diseases are bacterial in nature or responsive to antibiotic treatment. Additionally, the notion of a disease being non-contagious contradicts the fundamental aspect of communicability, while indicating a disease that occurs only in certain geographic areas describes endemic or localized illnesses rather than the broader scope of diseases that can be transmitted.