Parents are being asked to urgently get their children vaccinated against measles to stop an outbreak of the disease spreading.
Public Health Wales (PHW) said the number of cases linked to a Pontardawe school had risen to 13, while a case had been reported at a Swansea nursery.
This outbreak comes less than four months after Wales' biggest measles outbreak ended - centred on the same area with 1,200 suspected cases.
PHW said it was "very frustrated".
"We now have cases not only in Cwmtawe school but also in two primary schools, a further education college and a nursery," said Dr Jorg Hoffmann, PHW's consultant in communicable disease control.
"In all of these places there are unvaccinated children and young people who could easily catch and spread measles."
Health officials have already written to all parents of unvaccinated children in Cwmtawe School to urge them to arrange for their children to be vaccinated.
Pupils were offered the MMR vaccination with 55 children immunised during the school session but at least 46 did not take up the offer.
Dr Hoffmann added: "We are very frustrated to see more cases of measles in the area so soon after the large outbreak earlier this year.
"We are very keen for this to be stopped before it can get any bigger and we return to a position where children are admitted to hospital or die or are damaged by the disease.
"The only way to achieve that is through vaccination and I urge parents whose children have not received two doses of MMR to ensure that they speak to their GP immediately to arrange this quick, safe and effective vaccine."
The Swansea area epidemic lasted eight months with suspected cases first reported in November 12.
By the time the outbreak was declared over in July, it had resulted in 1,219 notifications of measles cases in the Abertawe Bro Morgannwg, Hywel Dda and Powys health board areas.
Some 88 people visited a hospital due to measles during the outbreak and more than 75,000 were vaccinated throughout Wales.
Gareth Colfer-Williams, 25, from Swansea, died from pneumonia after contracting the virus at the height of the epidemic.
The symptoms of measles include a fever, fatigue, runny nose, conjunctivitis and a distinctive red rash.