The Department of Health will conduct tuberculosis tests on some 300 to 400 students and staff at Central Falls High School, after the state learned that a student with an active case of TB had been attending school with the illness for several weeks.

The risk that anyone caught TB from the student is “low, but real,” said Health Director David R. Gifford.

The tests, which involve pricking the skin, will be conducted by Health Department nurses and school nurses at the high school starting today. Tests will be given to all juniors and seniors as well as people who spent extended periods of time in close contact with the student.

Each year, Rhode Island sees about 50 tuberculosis cases that are active and contagious, including occasional cases involving schoolchildren. What was different about this incident, Gifford said, is that the student had suffered from low-grade symptoms for “a couple of months” before he was diagnosed last Thursday. “It was evident that this individual had been in school the whole time,” Gifford said.

In what Gifford called “an abundance of caution,” the state decided to test all the juniors and seniors, not just — as is typical — those who’d had close contact with the sick boy. “Because we’re testing so many, we knew there would be a lot of concern and questions,” Gifford said, so the Health Department put out a news release and posted information on its Web site, www.health.state.ri.us. Letters were also sent home to parents yesterday.

Those tested will return to the nurse after two days to see if their skin has reacted in a way that indicates possible infection. Additionally, the skin tests will be repeated in 10 weeks because a person recently exposed to TB may not test positive for several weeks. If anyone does test positive for active TB, the state will provide treatment with antibiotics, which typically requires taking pills for several months.

The student is now recovering at home and will not be allowed to return to school until he is no longer contagious. But the school will remain open and parents are not advised to keep their children home.

Health officials emphasized that active TB shows up about once a week around the state, in every community and every ethnic group. The student in question had not been out of the country. “We don’t know where he got it,” Gifford said, but interviews with family members and others may eventually trace its origins.

Tuberculosis “occurs in every town, every age group, every race and ethnicity,” Gifford said. “There is no evidence that this has anything to do with immigration.”

Tuberculosis is a bacterial illness that spreads through the air when an infected person coughs, sings or speaks. Breathing air with TB can lead to infection. But TB does not live for very long outside the body, is killed by sunlight, and is not spread through ventilation systems. It also does not spread through food. It cannot be picked up from surfaces that an infected person touched, such as drinking glasses, eating utensils, cell phones or keyboards.

When exposed to TB, most people’s immune systems mount a defense that prevents the bacteria from multiplying or spreading. These people have “latent TB”; they don’t get sick and are not contagious. About 300 to 400 cases of latent TB are identified in Rhode Island each year.

But when a person’s body cannot fight the tuberculosis bacteria, the person has symptoms and can be contagious — an active case. Although TB can infect any part of the body, it most commonly settles in the throat or lungs. Symptoms can include unexpected weight loss, night sweats, weakness or fatigue, loss of appetite, coughing for more than three weeks or shortness of breath. It can be treated with antibiotics, but if left untreated for a long period of time, TB can cause serious illness or even death.

TB is a reportable illness. That means that when a doctor diagnoses a case of TB, he or she immediately informs the Health Department, which quickly sends someone to interview the family and test all close contacts. Those who are infected are treated at the RISE Clinic, the state-supported TB specialty clinic at 14 Third St., Providence. Nurses make sure that people complete their entire course of treatment, sometimes by observing them swallow the pill every day.

In the case of the Central Fall High School student, the doctor notified the Health Department on Jan. 3. When health officials went to talk with him they learned he’d had symptoms for some time, but not bad enough to stay home from school. Concerns arose that a large number of people had been exposed at Central High School, leading to the decision to test a large group.

The skin test involves injecting a small amount of a fluid just under the skin on the forearm. In people who have been exposed to TB, a bump will form. The test may cause redness or swelling, but that doesn’t necessarily mean a positive result. A nurse or doctors needs to measure the size of the bump to determine whether the test is positive.

Those with positive test results will be referred to the TB clinic for further medical evaluation and, if necessary, treatment. The state will pay for the evaluation and treatment at the RISE clinic.

Gifford said health officials do not suspect that the boy is carrying a drug-resistant strain of tuberculosis, and cases of TB that are resistant to multiple antibiotics have not been seen in Rhode Island. But every TB sample is tested for drug resistance at the state laboratory. Results will be available in four to six weeks.

Gifford said he didn’t know what the testing in Central Falls would cost, but said the money would come from the state’s tuberculosis program.

ffreyer@projo.com