Another outbreak, another costly reminder that the measures put in place to prevent the spread of COVID-19 are there for a reason.

State documents show that Maplecrest Rehabilitation and Living Center in Madison allowed an employee who failed a coronavirus screening to work a 10-hour shift, in violation of federal requirements for infection control and prevention, the Morning Sentinel reported Thursday.

The next day, that employee reported that she had previously come into contact with a person who was positive for COVID-19. The employee later tested positive herself.

There have now been at least 39 cases of COVID-19 and seven deaths at the nursing home, leaving residents and their families heartbroken and afraid.

It has all happened because of a failure to follow the rules that have been proven to keep us safe. These careless mistakes and failed leadership have spread the toll far and wide.

The state’s largest COVID-19 outbreak to date started Aug. 7 at the now-infamous wedding in Millinocket, where too many people gathered in too small an area, without face coverings.

The guests came from different parts of the state, and they brought the virus home. Still, the outbreak could’ve stopped there if others had followed the rules.

A staff member who attended the wedding carried the virus into the York County Jail, where Sheriff William King Jr. admitted to the Portland Press Herald that he had allowed staff and inmates — also against state and federal guidance — to refuse to wear masks. The jail was also not screening staff members upon arrival at work each day.

Nearly half of inmates and staff were infected, and the virus spread into the community, throwing the school year in nearby districts into turmoil.

A guest from the wedding also passed the virus, through another contact, to the staff member at Maplecrest, a certified nursing assistant who, according to state documents, said she was experiencing a cough, new muscle aches, repeated shaking with chills and a sore throat when she arrived at work on Aug. 11.

A charge nurse told the Department of Health and Human Services that it had been left up to individuals to let their bosses know when they were feeling ill, and that no one was reviewing the results of the screening tools.

No one can tell the future. But failing to require masks and properly screen incoming workers is as close as you can come to guaranteeing an outbreak in institutional settings, which since the appearance of COVID-19 have been recognized as one of the most vulnerable. Once the virus is introduced into such a setting, it spreads quickly, then follows workers home, where it worms into the community at large.

The colder weather now coming to Maine will force more people inside, into situations where the virus can spread more easily. The outbreaks at the Millinocket wedding, Maplecrest and the York County Jail show the high price of carelessness in such situations — and why it is so important for leaders, in public and private offices, to make sure the people around them are following the rules.