People are being urged to ensure they are protected against measles after six people have been confirmed as having the disease in Cardiff, Newport and Blaenau Gwent.
Two further probable cases are also under investigation.
Although cases are distributed across South East Wales, the first few cases are thought to have been exposed to an unknown highly infectious case in early February with investigations revealing that all were in the same location in Cardiff city centre on the same day.
Public Health Wales have now confirmed they are seeing the onward transmission in other parts of South East Wales from some of these early cases.
Public Health Wales is encouraging young adults and teenagers to check that they are up to date with the two doses of the MMR vaccine and are also urging parents to make sure their children are fully protected.
Children with early measles symptoms – which can include a fever, cough, runny nose and red eyes (conjunctivitis) - should be kept home from school.
The distinctive red rash develops two to seven days after these first symptoms and parents are being told to contact their GP or NHS Direct Wales.
About 1 in 5 children with measles can experience serious complications such as ear infections, pneumonia or meningitis. One in 10 children with measles ends up in hospital and in rare cases it can be fatal.