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A Centers for Disease Control and Prevention (CDC) case study released Thursday found that one ninth grade boy, who tested PCR negative before traveling, likely spread COVID-19 to over 100 people at a boys’ overnight summer school retreat in Wisconsin.

Among the 152 high school aged boys, counselors and staff members, there were 118 confirmed COVID-19 cases and 38 probable cases, the CDC said.

The tightly packed students, in session from July 2 to Aug. 11, didn't wear masks and shared dorms with beds close together.

The ninth grade boy who likely set off the spread had a negative PCR test less than a week before the retreat, but he developed symptoms the day after arriving, according to the CDC. A family member of the boy tested positive while he was at the retreat.

In order to lower the risk in this type of setting, the CDC said it is critical to implement pre-arrival quarantine and testing, symptom monitoring, early identification and isolation of cases, mask use, distancing and enhanced hygiene and disinfection practices.

ABC News’ Eric Strauss contributed to this report.

Liege, Belgium, is emerging as an epicenter of Europe's worsening coronavirus crisis.

In Liege, where there's a roughly 41% infection rate, the local hospital is at full capacity. ICU numbers have tripled in the last three weeks.

A doctor told ABC News that health workers who have tested positive for COVID-19 are still treating patients to keep the hospital system from crashing.

ABC News’ James Longman contributed to this report.

New York City's positivity rate on Thursday jumped to 2.7%, "a worrisome number," Mayor Bill de Blasio said.

"It is literally twice yesterday," he said. "We cannot allow that number to keep growing. We really have to double down."

The mayor said there's no particular reason for the uptick. The city's seven-day positivity rate average is 1.92%.

De Blasio repeated his message that "people really should not travel for the holidays unless it’s absolutely necessary.”

“Unfortunately, pretty much everywhere is doing worse at fighting coronavirus,” he said. “And if you go someplace else, the chance of bringing it back with you is high."

The mayor also warned New Yorkers to avoid large gatherings for Halloween.

"People want to trick or treat outdoors, small groups with masks on, that’s great. Big gatherings, parties, that’s not great at all,” he said, warning, "we find out about them, we are going to break them up."

ABC News’ Aaron Katersky contributed to this report.

Officials in Oklahoma are warning that COVID-19 hospitalizations in the Sooner State could triple by the end of December if people don't change their personal behaviors or if local ordinances are not enacted.

Projections from the Institute for Health Metrics and Evaluation showed that if local mask and social distancing mandates don't change, cases, hospitalizations and deaths will all increase in Oklahoma.

"We elected leaders to protect us, that’s one of their main responsibilities," Dr. George Monks, president of the Oklahoma State Medical Association, told ABC News affiliate KTUL in Tulsa. "To protect their citizens, you may upset a minority number of citizens in your district, but you’ve got to step up."

If mandates are eased, those numbers could go up even more, officials said.

From Oct. 16-22, the number of cases in Oklahoma increased 20.2% and the week-over-week deaths increased 34.5%, according to the Oklahoma Department of Health.

Monks said IHME projects said the state would reach its maximum need of 2,711 beds for COVID patients on Dec. 28. It doesn't have to be that way, he said, if everyone wore masks.

"It is projected we will reach our maximum number of new COVID-19 cases per day on 12/09/2020 at 5,005 cases/day," Monks said on Twitter. "This number changes significantly if we implement universal masking."

In Oklahoma, more than 119,000 people have been diagnosed with COVID-19, with at least 1,286 deaths.