More than 600 people—most of them children, aged one month to 15 years—have tested positive for HIV in the southern Pakistani town of Ratto Dero, numerous news outlets have reported.

While officials are still investigating the cause of the outbreak, health workers say that rogue doctors and unqualified people practicing medicine are behind the virus’ spread, mostly from using contaminated syringes. Such medical charlatans are popular in the area because they’re often cheaper and more accessible than trained medical providers.

Of the 607 or so confirmed cases in the outbreak so far, 75 percent are in children, according to reporting from the BBC. Cases started appearing in February when parents brought their children to doctors and hospitals with unexplained fevers that lingered. Once word spread that ill children were testing positive for HIV, parents began rushing their kids to a special camp set up by the health department at a local hospital.

This is at least the second HIV outbreak the province has seen in recent years. An outbreak in 2016 infected 1,521 people, mostly men. Health workers linked the spread to infected sex workers in the area. While the current outbreak largely affects children, health officials suspect that the two outbreaks are linked.

"I think the virus was being carried by members of the high-risk group [sex workers], and then lax practices by local quacks caused it to infect other patients," Dr. Asad Memon told the BBC. He’s the head of the Sindh AIDS Control Program.

A local doctor who specializes in treating children with HIV/AIDS told the BBC that children in the area typically get infected one of three ways: from a lactating mother who is HIV-positive, a contaminated blood transfusion, or contaminated medical equipment, such as syringes. Most of the children in the current outbreak have not had blood transfusions and have HIV-negative mothers.

So far, authorities have ordered around 500 dubious clinics in the area to shut their doors, and one local “child specialist," Muzaffar Ghangharo, was arrested earlier this month in connection with the outbreak. Authorities believe he spread the virus through dirty syringes. They’re still investigating whether he did so intentionally.

Ghangharo had denied the charges, noting that not all of the infected children were in his care.