Here are some examples:
* Malfunction: A failure to function properly.
* Malpractice: Negligence by a professional, especially a doctor or lawyer.
* Malady: A disease or illness.
* Malignant: (of a tumor or disease) tending to become progressively worse and to spread to other parts of the body.
* Malnutrition: A state of poor nutrition.
If you provide more context, I can give you a more specific answer.