These medicines are made in private and hospital pharmacies and used to treat everything from cancer to menopause symptoms to vision loss.
Often these products are name-brand medicines split into smaller doses, or drug combinations mixed from ingredients sold in bulk. That can easily lead to contamination if sterile conditions aren't maintained. The germ suspected in the current outbreak can spread in the air.
A shortage of many drugs has forced doctors to stretch supplies and seek custom-made alternatives if the first-choice treatment was not available.
The meningitis outbreak has killed four and sickened at least two dozen in five states.