Medical malpractice claims arise in one of two ways:

1. A medical professional's overt act--such as performing the incorrect surgery, or 
2. A failure to act--such as neglecting to recognize and treat a disease.

Importantly, a medical professional's error alone does not guarantee a successful medical malpractice claim. In a "failure to act" claim, the court will look at the medical professional's omission and determine whether or not a "reasonably prudent medical professional under similar circumstances" would have acted similarly. A rushed emergency room physician would be held toa different standard of care than a brain surgeon performing surgery. 

In order to prevail in a medical malpractice case, it is thus crucial to establish what standard of care the medical professional should be held to, and that they fell beneath that standard in treating the patient. This is typically established through expert witnesses in the same field as the defendant medical professional.