Which of the following is a common sign of illness in livestock?

Separation from the herd is a significant indicator of illness in livestock. Animals are social creatures and typically prefer to stay with their herd for safety and companionship. When a livestock animal isolates itself, it may signal that it is not feeling well. This behavior can stem from pain, discomfort, or other health-related issues. Monitoring herd behavior is crucial for early detection of illness, as a solitary animal may also reflect underlying health problems that could affect the entire herd if left unaddressed.

The other options, although relevant to animal behavior, do not typically signify illness. For instance, increased appetite and increased activity are generally seen as signs of good health and vigor. Similarly, social behavior reflects normal, healthy interactions within the herd, whereas separation indicates a deviation from typical behavior, raising concern about the animal's well-being.