It’s the result of a lightly regulated type of drug production that had a troubled past colliding with a popular treatment — spinal injections for back pain — used by millions of Americans a year.

The outbreak is thought to have been caused by a steroid drug contaminated by a fungus. The steroid solution was not made by a major drug company but was concocted by a pharmacy in Framingham, Mass., called the New England Compounding Center. Compounding pharmacies make their own drug products, which are not approved by the Food and Drug Administration.

On Monday, federal inspectors at the New England center found a sealed vial of the steroid afloat with so much foreign matter that it could be seen with the naked eye, FDA officials said Thursday. Under the microscope, the particles were a fungus.

The drug has been recalled, the clinics that used it are scrambling to warn patients who might have been exposed, and health officials are urging anyone feeling ill after a spinal steroid injection to contact a doctor quickly, especially for symptoms like severe headache, fever, nausea, dizziness, loss of balance or slurred speech. They are also urging doctors and hospitals to stop using any products made by the New England Compounding Center.

The pharmacy had shipped 17,676 vials of this potentially contaminated solution to 75 clinics, according to Massachusetts and federal health officials.

The question is, why? Why would pain clinics around the country rely on a pharmacy that mixes its own brand of unapproved drugs, especially for a delicate procedure like an epidural injection that has the potential — realized in these awful cases — to infect a patient’s nervous system?

There seems to be no one answer. Some doctors say compounding pharmacies offer specialty products or dosages not easily found elsewhere or sometimes simply better prices than big drug companies.

But other physicians have reservations about using compounded pharmaceuticals.

Dr. Arthur Watanabe, an interventional pain specialist in Spokane, said the steroid for spinal injection was available from mainstream drug companies, and he could not understand why anyone would buy it from a compounding pharmacy.

Compounding has been called an ancient art, something practiced long before medicines were premixed and sealed in bubble packs. The practice goes on today, allowed by states and the federal government, but it is supposed to provide custom-made products for individual patients with special needs.

It is frequently done in hospital pharmacies, and an estimated 2 to 3 percent of prescriptions in the United States are compounded prescriptions for individual patients, according to the International Academy of Compounding Pharmacists.

Pharmacies and regulators have struggled for years over what limits to set on the practice. Compounding has fallen between state and federal authorities, and the legal status of compounded drugs has never been clear.

Compounding is tolerated by the FDA, as long as the pharmacy does not ship across state lines, mass produce drugs or use commercial scale equipment, said James H. Ruble, an assistant professor at the University of Utah College of Pharmacy.

Health officials worry about compounding because it has its risks. The pharmacies are not held to the same safety standards as big drug companies. The current outbreak is not the first time a steroid made by this type of pharmacy has caused fungal meningitis in people receiving spinal steroid injections.

There were five similar cases, including a death, in 2002, all traced to one pharmacy in South Carolina that had been shipping its product to five states. Inspectors found contaminated vials and a complete lack of quality controls or sterility testing at the pharmacy.

In 2011, nine patients at an Alabama hospital died after receiving a feeding solution contaminated with bacteria, which had been prepared by a compounding pharmacy in Birmingham.

The New England center has had a long history of problems. Madeleine Biondolillo, director of the Bureau of Health Care Safety and Quality at the Massachusetts Department of Public Health, said there had been three complaints registered against it, in 2002, 2003 and this year in March, when the potency of a compound used in eye surgery was questioned. Its ability to perform sterile processing was also questioned.

State officials said the center was starting with a powdered drug and mixing it with liquid to make a solution of the steroid methylprednisolone acetate for spinal steroid injections.

But special care is needed in making drugs to be injected into the spine because of the seriousness of infections there, said Charles Leiter, the president of Leiter’s Compounding Pharmacy in San Jose, Calif.

“I try and stay away from drugs for the spinal column because if you make a mistake, they are dead,” Leiter said.