For months, aid agencies have feared the arrival of coronavirus in the world’s largest refugee settlement, the sprawling camps of Bangladesh’s Cox’s Bazar district, where about 855,000 people live in overcrowded and dirty conditions.

Now, the first case of COVID-19 has been confirmed in the nearby town, also named Cox’s Bazar. Aid workers worry that transmission into the camps is inevitable — and will result in a catastrophic outbreak.

Aid agencies are doing what they can to try to prevent or delay an outbreak and make preparations for when the wave hits. But the virus has overwhelmed the well-equipped health systems of wealthy Western and Asian countries, and officials know that Bangladesh, one of the poorest countries in the world, has no chance of responding to a rapid outbreak among one million refugees.

“I shudder to think of what an outbreak in the camps would be like,” Deepmala Mahla, Asia regional director for the humanitarian nonprofit CARE, told VICE News. “I would say this could be something we have never seen before.”

The camps, about 20 miles away from Cox’s Bazar town, are already among the most desperate places on Earth. About 855,000 Rohingya Muslims, who fled a wave of genocidal violence in neighboring Myanmar in late 2017, live in massive sprawls of overcrowded and makeshift shelters on uneven terrain, divided for administrative purposes into 34 camps. Another 150,000 or so Rohingya refugees live outside the camps, among the local community.

In the camps, basic measures to prevent the spread of the virus, like frequent handwashing and social distancing, are impossible, said Mahla. Families are packed into flimsy shelters at an average density of about 103,000 people per square mile. The traumatized and undernourished population rely on public toilets and wash facilities that are usually overburdened. More than half of households don’t have access to enough water to meet their needs, and only two-thirds have access to soap.

Juliet Parker, director of operations at Action Against Hunger UK, called Cox’s Bazar “a ticking time bomb.”

“When you look at the devastation [the coronavirus] has wreaked in so-called developed countries, the damage that could be done in fragile or conflict contexts could be devastating.”

Bangladesh is also just months away from monsoon season, which starts in June and brings high winds, flooding, and landslides that regularly wreak havoc on the refugees’ flimsy shelters and leave the camps mired in mud. The onslaught already makes the camps prone to outbreaks of disease.

“It’s very, very, very difficult,” Athena Rayburn, head of advocacy for Save the Children’s Rohingya response, told VICE News. “We’re doing what we can. But essentially we don’t have enough health care. We don’t have enough doctors. We don’t have enough [personal protective equipment]. And we’re looking at a situation where transmission would likely occur very, very fast.”

Aid workers have been planning for an outbreak on the assumption that once a case was confirmed in the district, it would spread locally and inevitably reach the camps. Now that scenario is beginning to unfold: On Tuesday, a 60-year-old Bangladeshi, a resident of Cox’s Bazar town who'd recently returned from Saudi Arabia, tested positive. The woman has been transferred to a hospital in Chittagong, and at least 21 people who came into contact with her have been quarantined.

“Looking at global examples, the idea that we will be able to escape any cases in the camps is unfortunately not realistic,” said Rayburn. “Our assumption — which is an unfortunate, but reasonable one to make — is that once it’s in the camps, nearly everyone will get it.”

In fact, COVID-19 may have already reached the camps; aid workers say no tests are being carried out to know for sure. Bangladesh, a country of 170 million people, has extremely limited testing capacity, and none are available in the camps yet. Across the country, only patients with possible symptoms and a recent history of travel are being tested. So far, Bangladesh has 39 confirmed cases, with four deaths.

Even if the tests were available, a stigma around the disease has already developed in the camps, meaning some refugees who experienced possible symptoms may avoid seeking treatment at clinics. Rayburn said, in the absence of testing, the best indicator that there wasn’t already a widespread outbreak in the camps was the fact that there hadn’t been a sudden spate of cases involving patients needing ventilators and intensive care.

The government has requisitioned three resorts to use as isolation facilities, and services in the camps have been cut back to the bare essentials to try to reduce the chances of humanitarian workers bringing the virus into the camps.

But they know this will be no match for the storm that’s coming.

“We’re in an unprecedented situation,” said Rayburn. “For the whole of Cox’s Bazar district, it could be pretty catastrophic.”