An unnerving case of a St. Paul nurse practitioner reusing syringes and exposing 161 patients to infection risk shows that unsafe injection practices still occur in U.S. health care. But the reasons remain a mystery.

In an era when contact lenses and mobile phones are disposable, researchers said they aren’t sure why any health care providers deviate from long-standing practice and reuse syringes. But they do know the consequence: More than 150,000 people have been tested in the United States since 2001 for exposure to viruses such as HIV and hepatitis C due to unsafe injections by health care providers.

“I don’t have a great answer for the why,” said Dr. Melissa Schaefer, a leading researcher of unsafe injection practices for the U.S. Centers for Disease Control and Prevention. “But there are no acceptable reasons for the why.”

Schaefer was surprised by her own national survey results last year, which showed that 12 percent of physicians believed that syringes had been reused in their facilities. Some providers might be trying to save money, she said, while others believe it is OK to use syringes if the needles are replaced, or if they don’t pull back on the plungers to draw infectious material into the cylinder.

Trouble is, infections have spread despite these faulty assumptions, Schaefer said. “They do not protect against the contamination of the syringe or make it safe for reuse. The risk is still there.”

Allina Health has tested 90 percent of patients who were exposed to infection risks at its St. Paul dermatology clinic by a nurse practitioner who started reusing syringes last October. So far, none appear to have infections that could have been transmitted via the syringes, an Allina spokesman said in a written statement.

While the risk is low, Allina officials will monitor these patients for months, because viruses such as hepatitis C can emerge slowly.

The nurse practitioner, who is no longer employed at the clinic, was first licensed as a registered nurse in 1978. She earned her NP license in 2014, according to records from the Minnesota Board of Nursing.

Why an experienced health care provider would reuse syringes is unclear. Calls to her home this week were not answered. The Allina statement said that personnel and privacy reasons prevented health system officials from publicly discussing why the practitioner reused syringes, other than to call it a “serious lapse in judgment.”

“This nurse practitioner took it upon herself, without consulting anyone else at the clinic, to reuse the single-use syringe as opposed to disposing of the medication,” the statement said.

Sometimes, veteran providers grow overconfident and cut corners, said Evelyn McKnight of Fremont, Neb., who created the HONOReform organization to spread awareness about unsafe injections after she was infected with hepatitis C in 2002 during one of the nation’s most egregious incidents.

“They’re always in a hurry, hurry, hurry, and there’s 10 people in the waiting room,” she said, speaking generally rather than about the St. Paul case. “There’s always pressure to keep up the pace.”

Ninety-nine cancer patients in Nebraska suffered hepatitis C infections, and six died from those infections, after nurses at the Fremont Cancer Center reused syringes and contaminated multidose bags of saline. McKnight said the saline bags at the clinic would start the day clear and end up cloudy. The nurse in charge lost her license as did the doctor, who fled to Pakistan amid an investigation.

McKnight worked with CDC officials to launch the One and Only Campaign to encourage health care providers to follow safe injection practices and to encourage patients to raise concerns.

Minnesota received grant funding from the campaign, which it used to provide on-site training for doctors and nurses until it created a webinar on safe injection practices to reach a broader audience.

The first online training occurred, coincidentally, in the same week in which the Allina case was publicly disclosed.

“It almost seems like common sense, that providers would use one needle, one syringe, only one time,” said Jacy Walters, who leads the health department’s training around safe injections. “However, that is not the case. And it’s those mistakes that lead to these situations.”

The CDC’s Schaefer searched news reports and state health investigations and found at least 35 incidents of unsafe injection practices between 2001 and 2011. Together, they required notification of more than 130,000 patients that the injections had put them at risk of infection.

Patients suffered hepatitis C infections in two-thirds of these reported incidents. Almost half of the incidents involved the reuse of syringes to draw medicine out of single-dose or multidose vials.

Schaefer is updating the research with more recent incidents. She now estimates that more than 150,000 people have been warned of infection risks since 2001.

“Unfortunately,” she said, “this has happened again.”