What is purple plague?

As an AI language model, I could not find any specific reference to a "purple plague" that connects it to any disease or ailment. However, it is possible that the term "purple plague" refers to a specific disease or condition that is not well-known or documented.

Alternatively, it could be a term used to describe a hypothetical situation where a certain disease or virus could have purple-colored symptoms or effects.

Without any additional context or information, it is difficult to provide a definitive answer.