Through a blue gate, they come for treatment in the early morning, faces wrapped in scarves against the cold. For now it’s a trickle, but their numbers are rising.

“I try to keep it secret, especially from my mother,” said a 26-year-old HIV patient at a foreign-run clinic in the Afghan capital, Kabul. “If she knew I had HIV, she would die.”

Shrouded in ignorance and taboo, HIV and AIDS have crept up almost unnoticed to join the long list of misfortunes visited on Afghanistan over the past several decades.

Officially, there are 636 cases in a population of about 30 million people. Health experts say the real figure is far higher, and growing.

Set alongside an almost decade-old war, desperate poverty and a government dependent on military and financial aid to reach beyond its cities, the fight against HIV and AIDS struggles to compete for the country’s meager resources.

Social stigma in a deeply conservative Muslim country has driven the disease underground and is complicating efforts to coordinate a response.

“Most U.N. agencies would say the number is in the thousands rather than the hundreds,” said Olivier Vandecasteele of Medecins Du Monde, which began anti-retroviral treatment in Kabul in 2009.

Harsheth Virk, an expert in HIV/AIDS prevention and care at the U.N. Office on Drugs and Crime (UNODC) in Kabul, said: “You have these pockets where it could potentially be explosive.”

Studies suggest Afghanistan currently has a “concentrated HIV/AIDS epidemic” - concentrated within a growing population of injecting drug users.

The country’s drug problem is well known.

The source of 90 percent of the global supply of opium used to make heroin, Afghanistan has around 1 million drug users.

Of those, an estimated 20,000 inject, and 80 percent of them share needles, many huddled in squalor under bridges or in abandoned buildings until they are rounded up and jailed or moved on by police.

The HIV sufferer, who spoke to Reuters on condition of anonymity, said he believes he contracted the virus when sharing needles in Iran. His account is a common one.

A majority of people registered as HIV positive in Afghanistan have lived abroad, mainly in neighboring Pakistan or Iran where rates of drug addiction are also high.