What prefix means "bad" or "painful"?

The prefix that means "bad" or "painful" is "dys-." This prefix is often used in medical terminology to indicate a negative or dysfunctional condition. For example, the term "dysfunction" refers to impaired or abnormal functioning, while "dysphagia" describes difficulty swallowing, which can be painful or problematic.

In contrast, "mal-" also conveys a sense of badness or poor quality but is not specifically connected to pain or discomfort. "Eu-" denotes something that is normal or good. "Homo-" refers to sameness or uniformity. Thus, "dys-" is the prefix that most accurately captures the concepts of being bad or associated with pain in a medical context.