Custom-mixed medicines like the steroid shots suspected in a deadly meningitis outbreak have long been a source of concern, and their use is far wider than many people realize.

These medicines are made in private and hospital "compounding pharmacies" and are used to treat everything from menopause symptoms and back pain to vision loss and cancer. Unlike manufactured drugs, these specialty products are not subject to approval by the Food and Drug Administration.

They're often drugs obtained from manufacturers that are split into smaller doses, or drug combinations mixed from ingredients sold in bulk. Any of those steps can easily lead to contamination if sterile conditions aren't maintained. For example, the fungus suspected in the current meningitis outbreak can spread in the air.