What is the origin of the idiom "bad blood" and when to use it?

The origin of the idiom "bad blood" is rooted in the belief in traditional medicine and folklore that associated certain physical and emotional conditions with the quality of one's blood. It was believed that having "bad blood" could lead to various health problems and even impact a person's temperament, causing them to be irritable or prone to conflict. This expression is commonly used in discussions related to personal relationships, family feuds, rivalries, and even broader societal or political conflicts where there is a history of animosity.