What was York County Sheriff William King thinking?

For months, by his own admission, King allowed staff and inmates at the York County Jail to choose for themselves whether to wear masks. Few did.

Now, in one of the state’s largest single-facility outbreaks of COVID-19, nearly half of inmates and staff are infected. The virus has spread throughout the jail and into the community.

York County officials are investigating the outbreak. The rest of us are left to wonder how an elected official could make such an obvious mistake. We are, again, reminded how quickly the novel coronavirus can spread from one place to another, ruining well-laid plans along the way.

The jail clearly failed to follow federal and state guidelines for congregate settings.

Besides making masks optional — until the first case was reported Aug. 19 — the jail was not screening staff members upon arrival at work each day. A staff member attended the Aug. 7 super-spreader wedding in Millinocket that has now been linked to more than 140 cases throughout the state, state officials say.

In an interview late last month with the Press Herald, King said he did not believe that the absence of masks contributed to the spread of the virus at the jail. “Hindsight is 20/20,” he said.

Except in this case, foresight was crystal clear too. Since the first case of COVID-19 was confirmed in the U.S., prisons and jails have been identified among the facilities most vulnerable to an outbreak. A partial survey released by federal officials all the way back in May found more than 7,500 cases and more than 100 deaths linked to correctional facilities.

The COVID-19 case and fatality rates are much higher in prison and jails than elsewhere, even though the incarcerated population is, as a whole, much younger than the population in general, and thus less susceptible to severe illness from the disease.

King ignored all those warnings. Like the organizers of the now-infamous Millinocket wedding, the sheriff put in place the conditions known to facilitate the spread of the virus.

All it took then was one of Maine’s few active cases to act as the match on the dry tinder. Now the state’s solid grasp on the disease looks a little more tenuous, and communities in two far-apart areas of the state are being forced to reassess how safe they are — and to rethink their plans for school reopenings, scholastic sports and other activities and events.

In the age of COVID, that’s how quickly a few poor decisions by individuals turn into a big problem for everyone.

The Maine Department of Corrections said it is now increasing oversight of the state’s 15 jails, asking each to write new policies on preventing the spread of COVID within their facility.

We hope that’s not necessary for the rest of the state’s jails. We hope they have been taking the necessary precautions all along.

Jail officials and other members of law enforcement certainly took the virus seriously in its first few months, as they brought the statewide jail population, most of whom are in custody even though they have not yet been found guilty of a crime, down from more than 1,600 in January to below 1,000 in May.

But the population was back up to nearly 1,400 last week, the Press Herald reported.

It’s likely that hundreds of those incarcerated Mainers could be freed without sacrificing public safety. There should be fewer arrests too. Along with requiring the universal use of face coverings, robust testing of inmates and staff, and other precautions, fewer inmates would make the jails safer.

The York County Jail didn’t take the necessary precautions, making an outbreak there all but inevitable. King should’ve seen it coming. He should’ve had the people under his authority and care follow the guidelines.

But he didn’t, and the effects of that decision will be felt throughout his jail, and the state.