An ague in the spring, Is physic for a king.

The proverb presents a bout of springtime fever as something that may do a person good, despite its discomfort. Calling it medicine fit for a king emphasizes the value it assigns to that supposed benefit.

It also expresses a way of looking at an unwelcome experience: what feels like a setback may prove helpful. The saying frames illness optimistically, rather than describing how to treat it.

Use it when speaking about a fever that occurs in spring and wishing to cast the experience in a hopeful light. It works as reassurance or as a reflection on an apparent misfortune, not as practical health guidance.