What does it mean to say that someone is someone's "white monkey"?

A disease.

Diphtheria is an acute respiratory infectious disease caused by Bacillus diphtheriae, which is characterized by congestion, swelling and gray-white pseudomembrane formation in the throat mucosa, and can cause myocarditis and peripheral nerve paralysis in severe cases. Diphtheria belongs to the category of febrile diseases in traditional Chinese medicine, and diseases such as throat obstruction, throat wind, throat lock wind, termite sore, diphtheria and diphtheria wind in traditional Chinese medicine literature all include this disease.