Montgomery County’s earliest sign of a whooping cough outbreak came during the first week of the school year, on Aug. 26, when health officials say they got a call about a newly diagnosed case involving a student who had attended a Pennsylvania summer camp.

During the next 36 hours, 11 more cases were identified across Montgomery. Then one surfaced in Fairfax County. By the second week of school, Montgomery had 15 cases among students from 9 to 18 years old, in both public and private schools.

What all but one of those cases had in common was a Jewish overnight camp popular with Washington-area families — Capital Camps — located near Waynesboro, Pa., roughly 60 miles from Rockville.

After the camp’s last session ended Aug. 10, camp officials learned that one teenage camper’s cough was more troubling than they knew: It was diagnosed by a doctor as whooping cough, a highly contagious disease that often spreads through coughing and sneezing.

The spate of cases has left Montgomery health officials working long hours to contain it. Letters have been sent home to parents at seven Montgomery public schools — most recently Walter Johnson High School in Bethesda — advising them to be on the lookout for symptoms.

Whooping cough, or pertussis, begins with cold-like symptoms, but it can lead to coughing fits that include a “whoop” sound at the end. Though treatable with antibiotics, the illness is most dangerous in infants and those with weak immune systems. Most children are vaccinated as infants, and a booster is recommended at age 11 or 12.

Nationally, whooping cough has been on the rise in recent years. This year, there have been more than 17,300 cases reported, a 30 percent increase compared with the same time a year ago, according to figures from the federal Centers for Disease Control and Prevention.

Alison Albert of the CDC said the disease is cyclical, with cases rising and falling, but noted that the recent increase might be the result of increased diagnosis and reporting or a move in the 1990s to a safer vaccination that might not be protective as long. She said research is underway on the vaccine’s effectiveness with a new strain of pertussis that health officials have seen in the past few years.

Albert said the outbreak related to the Pennsylvania camp does not appear surprising. The disease is not uncommon, she said, and vaccinations don’t provide endless protection.

“The sooner we get on top of these, the better we can limit the spread,” said Ulder Tillman, Montgomery County’s health officer, who urged parents of students starting seventh grade to make sure they have received a Tdap booster, which includes a vaccination against whooping cough. Maryland now requires the booster for all children in public and private schools.

Capital Camps chief executive Jonah Geller said immunizations are required for campers to attend the camp and said the camp was quick to follow the guidance of health authorities as soon as the diagnosis was made. “We care very deeply about the health and safety of every single camper and every single staff member,” Geller said.

Montgomery health officials confirmed last week that all of the campers who developed whooping cough were vaccinated against it. That they fell ill anyway is likely a reflection of the waning of protections from previous immunizations, which means students in school with those who were infected could still be at risk. The health department has not identified any cases spread by campers after they returned from camp.

But those vaccinations are important in any case, health officials said.

“It may have kept these kids from getting any sicker,” said Cindy Edwards, senior nurse administrator for communicable disease and epidemiology with Montgomery’s health department.

Montgomery had 40 whooping cough cases last year, but health officials in Montgomery and in the Maryland Department of Health and Mental Hygiene said they could not recall the last outbreak of associated cases.

“This is probably the first one we’ve had in a while, in terms of a number of cases associated with a specific event,” said Kurt Seetoo, immunization epidemiologist at the state health department.

At Capital Camps, based in Rockville, Geller said perhaps 50 percent of the 1,000 summer campers each year come from the Washington region, many of them from Montgomery. The student who was diagnosed with the disease saw the camp’s medical staff but did not show signs of whooping cough then, Geller said.

A Montgomery County father whose daughter developed the illness said the cough surfaced a week after camp ended. It sounded bad, he said, but at first no one — including the family’s doctor — thought of whooping cough because they did not know of any possible exposure. That changed after Aug. 25, when the family received an e-mail from the camp, which forwarded a letter from Pennsylvania health officials.

The family’s doctor then did a test, which turned out positive. Right away, the girl stopped attending school — she had gone to classes for just one day — and started a course of antibiotics, said her father, who spoke on the condition of anonymity in order to protect the girl’s privacy. She is now doing well, he said.

Thinking back, he said, “the cough did have that weird sound to it, that whooping sound.” The father said he does not blame the camp. “It’s something that happens,” he said. “People are going to be sick. It could have happened anywhere.”

The whooping cough case in Fairfax involved a student at Gesher Jewish Day School, which started classes Tuesday, eight days after Montgomery. By then, the student had been treated and was not contagious, said school spokeswoman Lisa Stern. Stern said parents were notified.

About a dozen of the school’s students attended the camp, and no one else was diagnosed, she said.

“We were fortunate in that it all came about before school started,” she said.

Other Washington-area public school systems said they had not seen cases of whooping cough this school year, including in Alexandria and Arlington, Fairfax, Loudoun, Prince George’s and Prince William counties. D.C. health officials have not seen a camp-associated outbreak and have had 12 cases since January.

In Montgomery and other areas, immunization clinics have been held to help get students vaccinated. Montgomery’s clinics will continue this week, as a deadline of Sept. 12 approaches to show proof of vaccinations.