Virtually all surgical procedures have an associated mortality and significant morbidity. Radiation and chemotherapy carry obvious harms. Psychotherapy can disturb some patients seriously. Admitting patients to a hospital exposes them to unusual and virulent pathogens. Even simple bed rest predisposes to thromboembolic disease.
Any intervention carries risk.
So any practitioner inevitably has patients who accrue harm as a result of his or her interventions. Furthermore, even doing nothing can result in harm, including failure to immunize, failure to treat many conditions, failure to pursue appropriate diagnostic procedures, failure to promote healthful behavior, and failure to heed the warning signs of a potentially suicidal patient. Above and beyond all this, we are human beings. Even the wisest, most competent, most dedicated, and most careful among us makes mistakes. So errors tend to be ignored, rationalized, minimized, denied, and presumably repeated as the IOM report asserts.