What defines an infectious disease?

An infectious disease is defined by the invasion of harmful agents, specifically pathogens such as bacteria, viruses, fungi, or parasites, which can cause illness in a host organism. The critical aspect of infectious diseases is that they are caused by these living organisms that can multiply and spread, leading to a range of health issues. This definition emphasizes the role of these agents and the process of transmission which is central to understanding how infectious diseases operate in populations.

In contrast, the other options do not align with the definition of infectious diseases. For example, a disease that only affects animals pertains to zoonotic diseases but does not encompass all infectious diseases. Similarly, a disease caused by genetics is classified as a genetic disorder rather than an infectious disease. Lastly, a disease that cannot be transmitted would be classified as non-communicable or not infectious, as infectious diseases are characterized by their ability to spread from one host to another.