A few weeks ago, I at­­tended a college re­­union. It was fun and refreshing to see friends from years ago. Although I returned home feeling fine, apparently the stress of traveling combined with being around a new group of people was too much for my immune system.

A few days later, I was sick. Luckily, it was a viral infection, and I quickly re­­covered, but not all illness clears up without medication. Both humans and animals get sick, and sometimes recovery requires antibiotics.

On the farm, the age of the animal, time of year, weather conditions, pen changes and other stressors can all contribute to the need to use...