One of the immigrants who is being detained at a federal prison in California has come down with chickenpox, according to a letter obtained by LAist. Detention facilities are usually unsanitary areas with poor health care, and it is unclear how many people have been exposed to the very contagious chickenpox virus, which spreads through the air and can take up to 21 days to develop into the rash.

Earlier this month, the Trump administration moved 1,000 immigrant detainees to California’s Victorville Federal Correctional Complex as part of its vow to crack down on unauthorized immigration. This is the first time federal prisons have been used to hold this many immigrants. On Monday, the warden sent a letter to over 800 prison staff members about the sick detainee, who has now been isolated. Staff are now cleaning the housing unit and visiting room, the Bureau of Prisons told LAist. But in an overcrowded, understaffed area housing thousands, one case could easily spread and cause widespread health problems.

It has long been known that disease spreads easily at immigration and detention centers. Many people who cross the border already need treatment for disease (like chickenpox, tuberculosis, or scabies), and sanitation is a big problem. Last Friday, prison staff protested the decision to house the detainees, claiming that it would make conditions more unsafe. John Kostelnik, president of the union representing Victorville workers, told the Los Angeles Times that the prison lacked the medical staff to deal with the detainees, and it needed additional doctors and nurses.

In 2014, the Department of Homeland Security’s inspector general released a report on the unsanitary conditions, noting that two Homeland Security workers had ended up contacting chickenpox and spreading it to their children. In 2016, The Guardian wrote about a case of a child in a detention facility in Pennsylvania whose highly infectious diarrheal disease went untreated for weeks, exposing at least 40 other children. That facility, the Berks County Residential Center, only housed 80 detainees, far from the 1,000 at the Victorville complex. Human Rights Watch has also stated that poor health care at immigration detention facilities is responsible for severe suffering, and sometimes the preventable death, of the people being housed there.