Such an outbreak is unusual among children, who typically contract HIV from their mothers during pregnancy, birth or breast-feeding. And as the number of cases grows, experts fear that the virus that can cause AIDS will spread.

“Initial investigations reveal that used syringes are being repacked, which may not only grow significantly the number of HIV cases but also other diseases,” Zafar Mirza, Pakistan’s top health official, said at a news conference last week. “The use of unsafe syringes might be one of the causes for spread of the disease, but the government is making all-out efforts to ascertain the exact cause.”

Concern that the virus was quickly spreading grew in April after parents in the city of Ratodero alerted local doctors that their children had fallen ill with fevers they couldn’t control, a common early symptom of HIV. When test results showed that these children, most of whose parents were HIV-negative, had contracted the virus, health workers were befuddled.

Then officials traced a number of the families’ steps back to a single doctor. Authorities arrested him in late April, accusing him of infecting dozens of patients with the virus through dirty syringes. Police later dropped an inquiry into whether he had done so intentionally, but he remains behind bars as the investigation continues.

The doctor’s lawyer told CNN that his client was not using infected syringes and “has been made into a scapegoat for the larger crisis in the region.”

As news traveled across Sindh province that many children had been infected with HIV, thousands of people poured into local health centers, asking to be tested for the virus. Soon, hospitals were overcrowded, Mirza said, and hundreds more people received positive diagnoses.

Nisar Ahmed, whose 1-year-old daughter recently tested positive for HIV, told Agence France-Presse that multiple children in his village also tested positive.

“I curse the doctor who has spread this disease to every child,” he said.

Since the outbreak began, officials have shut down clinics they deemed unsafe, as experts warned that the problem probably goes far beyond one doctor because syringes are commonly reused across the region.

“I find it hard to imagine it’s just one syringe or one doctor or just one batch of syringes,” said Werner Buehler, senior fund portfolio manager at the Global Fund to Fight AIDS, Tuberculosis and Malaria.

When hundreds of people receive diagnoses in such a short time period, “that looks like a widespread practice,” he said. And for the vast majority of them to be children is “exceptional.” This region of Pakistan has experienced HIV outbreaks before, but those have typically affected older populations and stemmed from high infection rates in sex workers and intravenous-drug users.

Reusing syringes in medical facilities is widely banned because needles can easily spread viruses such as HIV and hepatitis C between patients. Although it’s considered malpractice, syringes are often reused in Pakistan, “especially among the poor,” said Quaid Saeed, HIV/AIDS adviser to Pakistan’s National AIDS Control Program.

In late May, at the request of the Pakistani government, the World Health Organization deployed a team to the region to investigate the cause of the outbreak. The group will announce its findings later this month.

In the meantime, Mirza, the government official, said Pakistan has requested an additional 50,000 screening kits and plans to set up three new treatment centers in Sindh province, where the outbreak began.

A Pakistani doctor examines a blood sample from a woman for an HIV test at a hospital in Ratodero. (Rizwan Tabassum/AFP/Getty Images)

Even before this outbreak, Pakistan lagged far behind the United Nations’ ambitious “90-90-90” goal for HIV treatment and prevention globally. By next year, UNAIDS aims to have diagnosed HIV in 90 percent of infected people, provided 90 percent of them with antiretrovirals, and suppressed the virus in 90 percent of those treated.

But the agency has warned that the epidemic is expanding in Pakistan, with cases among young people increasing by 29 percent between 2010 and 2017.

Mirza said that a conservative estimate for the number of people infected with HIV/AIDS in Pakistan is about 163,000. But only a small fraction of them are registered with state-run assistance programs, in large part, he said, because the illness is “taken here as a big stigma, a shame.”

That most of the diagnoses in recent weeks have been in children poses another challenge: Limited amounts of child-friendly antiretroviral drugs are available because HIV is less common in children than in adults. Before this outbreak, the WHO had documented only 1,200 cases of children being treated with antiretrovirals in the entire country.

The uptick in HIV cases will come as a setback for Pakistan’s health officials.

For decades, Pakistan has battled the misconception that immunization for polio causes paralysis and infertility. Much progress has been made in acclimating once-resistant communities to that vaccine, which is administered through a mouth drop, with the number of polio cases decreasing from 20,000 in 1994 to 12 last year.