PORTLAND, Maine — A case of pertussis at a Portland private school is the latest instance in this year’s statewide upswing of the highly infectious disease, also known as whooping cough.

The Maine Center of Disease Control and Prevention recently determined that a middle school student at the Breakwater School had contracted pertussis, according to emails sent by a school employee to parents and staff and obtained by the Bangor Daily News.

The state CDC is investigating “a cases of pertussis [sic]” at the small Brighton Avenue school and parents are advised to watch their children for symptoms of the disease, school nurse Isaiah Meyer wrote in a Thursday email to parents

Pertussis often initially presents as a common cold but can eventually result in severe coughing spasms, vomiting and difficulty breathing. It is especially threatening to babies, according to Dr. Laura Blaisdell, a Yarmouth pediatrician.

“Pertussis is a highly communicable disease,” said Blaisdell. “It’s very annoying and severe for adolescence and can be deadly for newborns.”

The infection at Breakwater is in addition to the 129 pertussis cases that the CDC diagnosed statewide between Jan. 1 and April 30, compared with 56 cases during the same period last year.

Instances of the disease were most common in more densely populated southern Maine, with 74 cases between Cumberland and York counties, while Waldo county had the highest rate of infection relative to population.

While there is a vaccine for pertussis, even those who have been inoculated sometimes can still catch the disease.

When the rate of inoculation is high, even unvaccinated people are less likely to get sick, thanks to a phenomenon called community or herd immunity, according to Dr. Jonathan Fanburg, a pediatrician with Maine Medical Center. But where the vaccination rate is low, the disease can spread quickly.

“If everybody in the neighborhood is vaccinated you’re much less likely to catch it, but after [the vaccination rate] drops below a certain point it’s like wildfire and it just starts going,” said Fanburg.

At Breakwater the rate of vaccination against pertussis was far below the average for schools statewide last year, according a Maine CDC survey.

During the 2015-2016 school year, 77 percent of Breakwater kindergarten students were vaccinated against the disease, compared to a statewide total of 96 percent, according to the survey. At the school, 73 percent of first grade and 83 percent of seventh grade students were vaccinated, compared with 97 and 98 percent statewide.

These rates are affected by the relatively low number of students at Breakwater. For instance, the CDC survey of last year’s kindergarten class only included 13 kids. The school did not respond to requests for comment.

The rate of students vaccinated for pertussis in Portland’s various public schools had a percentage generally in the mid-nineties, with some in the low-nineties or high-eighties. Overall, Maine has one of the highest youth vaccination rates in the country.

Breakwater has about 150 students, according to its website, and despite the small population Blaisdell said that the relatively low rate vaccination is worrying.

“Clearly this percentage is way below what we need to achieve community immunity,” she said.

Meyer, the school nurse, instructed Breakwater staff to send students displaying symptoms home. Students should stay out of school until they’ve been on antibiotics for five days, or for 21 days “if parents choose not to use antibiotic treatment,” he wrote.

“Vaccinated children can still acquire the disease and unvaccinated students are not to be excluded from school unless symptomatic,” Meyer told staff.

More information about pertussis is available on the U.S. CDC website.