What best defines a zoonotic disease?

A zoonotic disease is best defined as one that can be transmitted between animals and humans. Understanding this definition is crucial, as it underscores the interconnectedness of animal health and human health. Zoonotic diseases can arise from various pathogens—such as viruses, bacteria, and parasites—that are capable of crossing species barriers. This category includes well-known diseases like rabies, avian influenza, and certain strains of salmonella.

The other options do not accurately capture the definition of zoonotic diseases. For instance, a disease affecting only livestock or being specific to farm animals would not encompass diseases that can spread to humans from wildlife or pets. Similarly, a disease passed only from human to human lacks the essential characteristic of zoonotic diseases, which is the involvement of an animal reservoir. Thus, the definition of zoonotic diseases is significant in both veterinary and public health contexts, illustrating the importance of monitoring animal health to prevent potential outbreaks in human populations.