What is defined as an illness transmitted directly or indirectly from an infected host?

The term described in the question refers specifically to an illness that can be passed from one individual to another, either through direct contact or through indirect means such as the environment or vectors. A communicable disease fits this definition because it encompasses a broad range of infectious diseases that are capable of being conveyed from person to person.

In understanding this concept, it's important to recognize how communicable diseases can spread through various forms, including airborne transmission, bodily fluids, or contaminated surfaces. This direct or indirect transfer is a defining characteristic that sets communicable diseases apart from other types of illnesses, which may not have a transmissible aspect.

Other terms in the question have distinct meanings. The incubation period refers to the time between exposure to the infectious agent and the appearance of symptoms. An epidemic refers to the occurrence of a disease that is greater than what is normally expected in a population, which may include both communicable diseases and non-communicable ones. A communicable condition isn't as widely recognized in public health terminology as "communicable disease," making it less precise when discussing transmissible illnesses.

Therefore, "communicable disease" is the most accurate and comprehensive answer, clearly aligning with the established definitions used in public health contexts.