First, they nixed the nachos. The Harvard College students thought finger food would be a bad idea for the formal dance they were planning earlier this month, so they chose items amenable to forks and tongs — spring rolls, pigs-in-a-blanket. And they made sure to have an abundant supply of cups, to prevent reuse.

The students weren’t worried about impropriety. They were worried about germs. Because this year, Harvard got the mumps, an old viral illness that crept back into campus life.

Since February, the virus has sickened more than four dozen people at Harvard, mostly undergraduates, in what is thought to be the school’s first mumps outbreak since 1937. Although no one suffered serious illness, many have had to spend days in isolation, and 22 people were still isolated Friday, the university said.

Students say the outbreak has been a frequent topic of conversation and jokes. A student blogger lightheartedly advised partygoers to check for the illness’s telltale swollen glands before kissing someone.

But for Henry M.N. Brooks, an 18-year-old freshman, mumps was not a theoretical worry. He was diagnosed with the illness in early March and, though his symptoms were mild, the experience left a lasting impression. To avoid infecting others, Brooks had to move into the Harvard Inn, a former hotel on campus now used as surplus housing — and this spring, as an isolation ward.

Except for one quick trip to get a blood test, wearing a face mask, Brooks didn’t go outside or see another person for five days. He didn’t even know who else was in isolation alongside him. When he ordered food, he was instructed to wait two minutes after it was delivered before opening his door. “Then I would open the door and find an empty hallway and a tray,’’ he said.

With computer and books in hand for the first two or three days, Brooks busied himself studying and learning the lines to a play he was in. “By day four, it was really difficult to focus,’’ he said. Being indoors, inactive, and especially never seeing other people “was strange and uncomfortable.’’

Although born into the digital generation, although able to text and e-mail, Brooks still sorely missed face-to-face contact with others. Now that he’s healthy, he said, he places a higher value on his time with friends. Even after a mere five days alone, “you come out with a fundamentally different sense of yourself,’’ Brooks said. “The human animal needs social interaction.’’

The outbreak started in mid-February, when an unusual number of students appeared at Harvard University Health Services with swollen faces and other symptoms that suggested mumps, said Susan Feinberg, spokeswoman for the Cambridge Public Health Department. On Feb. 29, mumps was confirmed in an undergraduate and a divinity student, Feinberg said.

The outbreak grew steadily, undeterred by spring break in March or by Health Services’ frequent entreaties to wash hands, avoid sharing cups, and seek care when sick. The university put stickers on water fountains, urging users not to share water bottles or touch a bottle’s mouth to the spout.

By Thursday, 50 cases had been confirmed.

Massachusetts health officials said that all the students had been vaccinated, so resistance to being vaccinated was not a factor. But the mumps shot is effective only 88 percent of the time. So every now and then, the virus can get a foothold among the 12 percent for whom the vaccine did not provide protection, especially if they live in close quarters — such as a college dorm. In 2013, 39 confirmed and suspected mumps cases occurred at Boston College.

In most years since 2000, fewer than 10 mumps cases were reported in Massachusetts. In 2013, there were 71. As of April 29, health officials had received reports of 85 likely cases for 2016, but not all have been confirmed. Although other universities have had cases, the only significant cluster in Massachusetts is at Harvard.

Universities in other states, particularly Indiana, have also experienced mumps outbreaks this year.

When the cases at Harvard reached 40 and the outbreak was trending on Facebook, Melissa Hammer was surprised to get urgent texts from friends back home in Hicksville, N.Y., worried that she was sick. “They thought it was much bigger than it was,’’ said the 20-year-old junior. Less than 1 percent of the university’s 6,700 undergraduates got sick.

Still, it’s an ever-present concern, Hammer said. “I definitely try to be more careful with washing my hands and drinking out of only my cup, and making sure everything is clean,’’ she said.

Matthew DiSorbo, a 21-year-old junior from Burlington, Conn., and one of those who planned the nachos-free dance, agreed that mumps is on students’ minds. “People are worried and trying to prevent it, but I don’t think it’s consuming or something causing undue anxiety,’’ he said.

Nick O’Brien, a 21-year-old junior from Springfield, said that mumps “hasn’t been too serious an issue on campus. It’s a very small part of the student population that’s been affected.’’

Still, O’Brien has noticed unusually heavy use of hand sanitizer in the dining hall.

Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer