What is the primary characteristic of a contagious disease?

The primary characteristic of a contagious disease is that it can be transmitted directly or indirectly from an infected animal to a healthy one. This means that the disease can spread through direct contact, such as when an infected animal touches or interacts with a healthy one, or indirectly via contaminated surfaces, objects, or the environment. Understanding this transmission dynamic is crucial for controlling infectious diseases in animal populations, especially in laboratory settings, where preventing outbreaks is essential for research integrity and animal welfare.

The other options do not fully capture the essence of contagious diseases. For instance, while some contagious diseases are indeed caused by viruses, they can also be caused by bacteria, fungi, or parasites, making the first choice incomplete. The third option suggests that contagious diseases only affect humans, which is not true, as many contagious diseases can affect both animals and humans. Lastly, although many contagious diseases do present symptoms, not all infectious diseases lead to noticeable symptoms in the host, especially during the incubation period, which makes that option inaccurate.