More than 170 pupils at two schools in Swansea have been offered a TB test after it emerged a member of staff has the infection.
Parents said they were concerned for their children
The National Public Health Service (NPHS) said the teacher at Gorseinon Infant School has been off work since the end of March.
The NPHS says the likelihood of TB spreading within Gorseinon infant and junior schools were "very low indeed".
Her three children, who also attend the schools, were diagnosed on Monday.
One mother who has children at Gorseinon infant and nursery schools said the tests should be carried out as soon as possible.
Natalie Davies said her youngest son Thomas had a history of chest problems.
"He's had pneumonia three times, so he has got a weakness, and he's got asthma as well.
"It is quite worrying, because they all play together, and it's quite a serious illness."
Dr Mac Walapu, consultant in communicable disease control with the NPHS, said: "It is only spread by people in very close contact with each other.
"However, as a precautionary measure and in line with guidance, we are screening those who may be at risk."
He added that all four were responding well to treatment and would return to school when better.
The Gorseinon schools have 31 staff and 337 full time and 72 part time pupils.
Tuberculosis (TB) is an infection caused by bacteria
It is usually found in the lungs but any part of the body can be affected and occurs in all countries and at all ages
TB is treatable with a combination of antibiotics and can be completely cured
The most common symptom is a persistent cough for more than two weeks, with spit that can sometimes be blood-stained.
TB is quite difficult to catch and can only be passed on by breathing in the germs spread by someone else
Source: NPHS Wales
Of these, 172 children are being offered tests.
All staff members who came into contact with the teacher and class contacts of her children have been sent a letter inviting them for a screening test on 9 May.
Parents have also been sent a fact sheet.
Cases of tuberculosis are increasing, according to official figures.
Provisional data from the Health Protection Agency show a rise of 2% from 2005 to 2006 to 8,171 cases across England, Wales and Northern Ireland.
London continues to account for the highest proportion of cases - 42%.