Screening of 172 pupils in two Swansea schools for TB is underway after a member of staff and her three children were diagnosed with the infection.
Pupils at Gorseinon Junior School are among those screened
Pupils at Gorseinon Infants and Junior Schools are being offered a skin test for the disease.
The National Public Health Service for Wales (NPHS) said the chances of the infection spreading within the school were very low.
The four diagnosed with TB are said to be responding well to treatment.
The teacher at Gorseinon Infants School has been off work since the end of March, while her children were diagnosed with TB last week.
Mac Walapu, of the NPHS, said the screening for TB was normal procedure in such cases.
TB can be spread from person to person
He said: "The test involves injecting a small amount of purified sterile protein just under the skin in the forearm.
"The test can be read two to three days later. We will be seeing every tested child later this week to read the reaction.
"A positive test does not necessarily mean the child has TB. A positive result can be caused by a reaction to a previous BCG vaccination or to exposure to TB at any time in the past.
"It can also be caused by exposure to other germs similar to TB which are mostly harmless."
The NPHS said there would be no final results available this week and children at the two schools would be monitored until the end of June.
Dr Walapu added: "We have had very good support from the education authority and staff in the school. Parents have also been very co-operative and supportive."