“This unknown enemy that we fight is real,” he said at his first news conference after leaving his sick bed. “None of us have had to deal with anything like this.”

Officers patrolling the streets and investigating crimes said the virus had ratcheted up stress and disrupted all the standard rhythms of police work. Instead of roll call, officers get temperature checks and an envelope with the day’s orders. They give arrested people masks and wipe down patrol cars after every encounter.

“I have to come into work concerned about whether I’m going to be the next victim or not,” said Officer Marc Perez, fresh out of the police academy, after a patrol shift through Northwest Detroit. “There’s only so much an officer can do to prevent himself from coming into contact with that actual virus. Every day is stressful for me.”

Detectives who are used to sitting across a table from suspects are now doing interviews on the phone, unable to read clues like body language, mannerisms and facial expressions, said Lt. Rebecca McKay, an investigator with the major crimes unit. Even in-person interviews at the jail now have to be distant, behind glass.

“It’s hard to build a rapport or any kind of relationship with an offender,” she said, “when you have a glass between you, and you have to yell back and forth to get a statement.”

At the best of times, Detroit wrestles with its crime rate. The city and its police force have improved markedly since Detroit declared bankruptcy in 2013. At the time, the police took so long to respond to 911 calls that some people didn’t bother phoning.