More than 200 pupils at a Flintshire secondary school are being tested for TB after a member of staff was diagnosed with the infection.
Health officials say the tests are a precautionary measure
Health officials confirmed the case at Holywell High School in June, after the staff member went off sick on 24 May.
Screening involves a skin test, carried out at least six weeks after the last contact with an infected person.
The National Public Health Service of Wales said the tests were a precaution, and the risk of spread was "very low".
Spokesman Chris Lines explained that around 250 pupils would be tested.
"The people we're inviting for screening are those pupils who have had close contact with this member of staff - that means having been in a class room on a regular basis," he said.
Screening is carried out at least six weeks after the last contact to ensure readings are as accurate as possible.
The results should be available within days.
The case at Holywell was the second confirmed TB outbreak at a school in Wales in just a few months.
In May, screening was offered at Gorseinon Infants and Junior Schools in Swansea, after a teacher and her three children were diagnosed.
Despite some calls for an inquiry, Wales' chief medical officer has said there are no plans to conduct a review, but said it was important to remain "vigilant".