More than two hundred pupils and almost a hundred staff will be offered screening for tuberculosis (TB) today at Ysgol y Strade school in Llanelli, as a precaution.
It follows screening last month which revealed positive test results in three pupils and a member of staff.
Experts say the risk of spreading the infection is low, and there is believed to be no ongoing risk within the school.
Sion Lingard, Chair of the Incident Control Team said:
"We are emphasising to pupils, parents and staff that it is rare for TB to be transmitted within a school environment.
"Further screening is being carried out due to the very small risk that others may have picked up the infection within the school. All those who are likely to have had some form of contact with the infectious staff member will be screened."
Dr Chris Williams, a Consultant Epidemiologist at Public Health Wales, has urged people with a fever or cough to think about going to their doctor - but tuberculosis is treatable, and it's "not something that people should be too worried about."
Public Health Wales says it is notified of around 200 cases of TB each year in Wales.
It says it is "very rare" for tuberculosis to be transmitted within a school environment, and screening is being offered to pupils as a "precaution."
We are emphasising to pupils, parents and staff that it is very rare for TB to be transmitted within a school environment and so we are offering screening as a precaution.
Close contact with an infectious individual over a long time period, such as living in the same household, is usually required for a person to become infected with TB.
The infection is difficult to catch but treatable with antibiotics.
Other close contacts of the person who has been unwell have already been traced. Another four cases of TB have been identified in the Llanelli area over the last year, but none of these have links to the school.
Investigations into these cases are continuing with the involvement of Hywel Dda Health Board with treatment, advice and screening offered where appropriate.
– Sion Lingard, Acting Consultant in Health Protection for Public Health Wales,