What defines a communicable disease?

A communicable disease is defined as a disease that can be transmitted from one organism to another, typically through various means such as direct contact, airborne pathways, or vector-borne transmission. This characteristic is central to the understanding of communicable diseases, as it highlights the mechanisms through which these diseases spread among individuals or populations.

The other choices do not accurately define a communicable disease. A disease with no known cause does not describe how it is transmitted; a fatal disease relates to its outcomes rather than its transmission; and a disease existing only in laboratory settings is not relevant to the concept of communicability in real-world scenarios. Hence, the defining feature of communicable diseases is their ability to be transmitted between organisms, making the selected answer the most fitting.