Whooping cough was first reported at the school Monday, Salt Lake County Health Department spokesman Nicholas Rupp said, and by Friday as many as 14 cases were being investigated.
About 30 unvaccinated students have been "excluded," or told to stay home, for a period of 21 days or longer if the outbreak continues, Rupp said.
"They're going to have to stay out as long as the outbreak is existing in their classroom and their school," he said.
Whooping cough is a vaccine-preventable infection that carries symptoms similar to a severe cold. It is highly contagious, and poses the greatest risk to infants and individuals with chronic respiratory conditions or compromised immune systems.
Rupp said pertussis is relatively common in school settings, but he added that the outbreak at AISU appears to have been accelerated by a large number of unvaccinated students.
Utah law requires parents to either vaccinate their children or declare an exemption to enroll in public schools.
Rupp said Salt Lake County recommends a pertussis vaccination rate between 92 percent and 95 percent to achieve disease resistance within a population, known as herd immunity. AISU's vaccination rate, he said, is roughly 89 percent.
"It's likely that their relatively high exemption rate is one reason that this outbreak is larger than we normally see in schools," he said.
AISU enrolls more than 1,400 students in kindergarten through 12th grade, Casaday said.
The charter school is also combined with a private school that enrolls several international students, he said, which creates some discrepancies in vaccine rates and reporting.
"Their requirements in their home countries are different than ours," he said.
The outbreak, Casaday added, appears to have affected adult faculty members as well, including some who had previously been vaccinated for pertussis. He noted that he was experiencing cold symptoms and had visited a doctor as a precaution.
"I'm starting a course of antibiotics today," he said.
The timeline of the outbreak also presents a challenge for the school, Casaday said. High school students have already completed their required annual testing, but SAGE tests for elementary and junior high students are scheduled to begin shortly after classes resume April 17.
"At the end of the year, you've got so much to do," Casaday said. "And now we've got this."
Rupp added that a week with no classes may aid in halting the spread of the outbreak.
"Having spring break in there may help us," he said. "They won't be able to infect each other and hopefully those who are affected by this will stay home and not be out in the community."
Tribune reporter Kelly Gifford contributed to this article