The state Department of Health is offering testing and monitoring at a Delaware County school where a student reported being exposed to tuberculosis and 20 others tested positive for a latent form of the disease.

Last week, Chester Upland district officials mailed letters to parents of the 293 students who attend the Science and Discovery High School, informing them that a classmate had reported being exposed to TB outside the school environment, according to district officials.

Permission slips for TB testing, administered last Friday, were included.

The results: 20 students among the 200 who opted for the exam tested positive for latent TB, an indication that they had been exposed to the germ at some point. Such results are not uncommon; between 5 and 10 percent of the population has latent TB, state officials said.

Joel Avery, a school district spokesman, said there were no active cases of the disease among students.

Phyllis Britz, district nurse administrator with the state Health Department, said that in most cases, there is no risk a person with latent TB can infect others.

"Even if someone develops the disease, it is very curable," said Britz, who works in the department's southeast office, in Chester.

Britz said all 20 students who tested positive appeared to be healthy and would be offered medical follow-up, including a chest X-ray and antibiotics.

"Most people don't know they have TB at the beginning," Britz said. The disease is transmitted when the germ is airborne, such as through a cough or when someone is talking or singing, she said.

Nationally, the Centers for Disease Control and Prevention says, the number of TB cases continues to decline, falling from 26,283 reported in 1991 to 11,545 last year.

The CDC recommends that people with the latent form of TB receive treatment to reduce the risk the infection could progress.

In 2009, there were 18 reported cases of TB in Bucks County, six in Chester, 10 in Delaware, and 20 in Montgomery, according to state officials. Philadelphia reported 97 cases. Each year, state health officials said, they monitor about 2,000 people with latent TB.

Occurrences of TB are rare but not unheard of in area schools. In 2000, a case was reported at Upper Darby High School; in 2001, a student at Furness High in South Philadelphia was stricken.

Joyce Wells, acting superintendent of the Chester Upland district, told a Dec. 15 school assembly about plans for the TB testing.

"The students were calm and comfortable," she said. "No one was rattled by the situation."

The district will offer more tests in January. Parents who want their children to be tested over the winter break may call the state Health Department at 610-447-3250 to make an appointment, the district said.