The number of cases of measles in an outbreak in Neath and Swansea has more than doubled in the last two weeks.
Public Health Wales (PHW) renewed its call for parents to urgently get their children vaccinated to stop the disease spreading.
PHW said it was "very frustrated" with cases reaching 36 since early October.
The latest outbreak comes less than four months after Wales' biggest measles outbreak ended - centred on the same area with 1,200 suspected cases.
The outbreak has affected four schools in the Abertawe Bro Morgannwg University Health Board (ABM) area.
"Parents and young people should not underestimate how serious measles can be and how quickly it can spread," said Dr Jorg Hoffmann, consultant in communicable disease control for PHW.
"In a school setting, one child with measles sitting in a classroom for just one hour will lead to at least 70 per cent of other pupils who are not vaccinated catching measles.
"To prevent this outbreak from spreading even further, it's crucial that unvaccinated children and young people receive two doses of MMR urgently and that those with symptoms do not attend school."
PHW is working closely with ABM to bring the outbreak under control and where a sufficient number of children are unvaccinated in a school where there has been measles cases, school vaccination sessions will be arranged.
The Swansea area epidemic lasted eight months with suspected cases first reported in November 2012.
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.
Although more than 70,000 catch up doses of MMR were given across Wales during the outbreak, around 30,000 children and young people in the 10 to 18 age group remain unprotected.
Dr Hoffmann added: "Parents who have decided not to vaccinate their children are not only risking their children's health, but are putting other children at risk, children either too young to be vaccinated or with medical conditions that prevent them from being vaccinated.
"We are very frustrated to see more cases of measles in the area so soon after the large outbreak earlier this year and 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."