Fairfax County health officials have suggested that at least 1,400 more faculty, staff and students at Lee High School in Springfield get tested for tuberculosis after more people have been found to be infected with the disease, Fairfax Health Department officials said Monday.

The tuberculosis investigation at Lee traces back to three active cases discovered there last school year. TB, as the disease is known, is a bacterial germ that attacks the lungs and can be deadly if not treated quickly.

Fairfax County Health Director Gloria Addo-Ayensu said the investigation had determined through skin tests that more people at Lee High have latent TB infections, a noncontagious form of the disease. She declined to say how many people have latent infections, but school officials estimated the total number is between 15 and 25.

“Although testing is ongoing, findings of a higher-than-expected positive skin-test rate among those tested so far has prompted the Health Department to expand our screening and testing to all students, faculty and support staff that were part of Lee High school during the 2012 through 2013 school year,” Addo-Ayensu said, noting that if not treated with antibiotics, latent infections can progress to the more dangerous active disease. So far, no more active cases have been found at Lee, she said.

In mid-June, the Health Department asked that 400 students and 30 staff members who were at the highest risk be tested for the disease. About a week later, health officials asked to test another 60 people, including students, staff and families from other Fairfax County schools who might have come in contact with the original three cases through extracurricular activities.

In all, about 60 percent of the original 490 — close to 300 people — have received the free tests.

Addo-Ayensu said that those who were tested were split into two categories: those who were born in the United States and those who were born in foreign countries. About 1 percent of native U.S. residents would normally have the latent infection at any one time.

Of those who have been screened in Fairfax, she said, about 5 percent of those born in the United States had a positive test for the latent infection, a rate that “was significantly higher than you would have expected.”

The Health Department would not disclose the exact rate of infection for those tested who were born outside the United States. Spokesman Glen Barbour said that rate was somewhere between 5 percent and 33 percent, the rate worldwide for latent infection.

Jane Moore, director of tuberculosis control and prevention for the Virginia Department of Health, said that five to 10 TB cases are investigated in schools every year. Fairfax County alone was responsible for about one third of the state’s 235 cases last year, she said. Northern Virginia, with a diverse immigrant population and workers who travel frequently for their jobs, is responsible for more than half of all cases in the state, she said.

Thomas R. Navin, who oversees TB outbreak investigations at the Centers for Disease Control and Prevention, said that the CDC “is not involved in the investigation occurring in Fairfax County but is monitoring the findings with the state health department.”

It is still unclear how the active cases were introduced to the Lee High population and how far the infection spread. Some families with students at Lee High said they think that a larger percentage of the school population should have been tested sooner, and some said Health Department officials turned away students who had not been specifically identified for testing when they showed up for initial screenings.

“I would have wished that they would have let the parents who came to them get their children tested, but [the Health Department] chose not to,” said School Board Vice Chairman Tamara Derenak Kaufax (Lee). “I know that people are frustrated. . . . I’m not a physician. But they are handling it in the best way they know how, given what they know.”

Ellen van Aartrijk said she and her daughter Alexa, then a senior, were among those the Health Department turned away.

“I felt that they have dropped the ball,” van Aartrijk said. “They were very dismissive. I thought, if this was your child, wouldn’t you want them to get tested? . . . Why wouldn’t you test them all? Especially when you don’t know who has been in contact with who.”

Addo-Ayensu said that only those who were identified as being at the highest risk were allowed to be screened at first. She said that the Health Department decided to expand the free testing to everyone “in an effort to minimize the anxiety and concern that comes with each public announcement of an expansion and out of an abundance of caution and so that everyone who may have been exposed is identified sooner rather than later in the summer when school is about to start.”

The free screenings are scheduled to begin Aug. 3 at Lee High School. Classes are scheduled to begin Sept. 3.