Forty-Three more people have tested positive for the coronavirus at the Union Rescue Mission, Los Angeles' oldest and largest homeless shelter. The sudden spike in cases comes despite the extreme precautions the shelter has been taking to prevent such an outbreak.

“We started early, getting ready for this to avoid it, and it has just come in like a roaring lion,” Reverend Andy Bales, CEO of the Union Rescue Mission, told the I-Team.

Just blocks from the Mission Monday morning, the LA Fire Department set up a pop-up testing station, where workers in hazmat suits handed out testing swabs to the homeless from behind a protective window.

The goal is to test a large swath of Skid Row’s 5,000 homeless, most of whom live on the sidewalks of this 50-block area of Downtown LA. But hours after the testing station was open, only a handful of people were lining up to get tested.

The first person in line for a test, who didn’t want to give NBCLA his first name, said a large number of the homeless population suffers from mental illness and many still don’t believe COVID-19 is a threat.

“They don’t even think people are dying. They don’t think people are getting sick, and that’s part of mental illness. They don’t want to believe the truth,” he said.

Results from Monday's tests will be available in 24 to 48 hours. Then comes the hard part: outreach workers from the LA Homeless Services Authority will have to track down and notify those who tested positive, even though most don’t have addresses.

At the Skid Row testing site, firefighters were on standby to transport anyone with flu-like symptoms to a nearby motel that was set up as a temporary shelter, to isolate them from other homeless people.

"Those that are symptomatic, we will get them sheltered, get them inside, get them safe," Los Angeles Police Department Chief Michel Moore told the I-Team.

At the Union Rescue Mission, anyone with symptoms or who has tested positive is being moved to a large tent on the property, which usually acts as a women’s shelter.

The Mission has also moved another 178 of its most frail residents offsite to protect them from infection. The shelter, which typically houses close to 1,000 homeless people, now has about 414 residents.

All told, 42 residents of the Mission have tested positive for COVID-19 in addition to one worker.

“This monster, this ferocious, mysterious disease has moved in and changed, everything,” Reverend Bales told NBCLA.