Public health officials said Friday they were investigating the first suspected death from measles in Britain in five years, after an outbreak blamed on a campaign against vaccinations.
More than 800 people have contracted the highly contagious disease in Wales in the past six months, centred around the southern city of Swansea.
Marion Lyons, director of health protection for Wales, said it had now been confirmed that a 25-year-old man from Swansea who died on Thursday had measles.
Health authorities were working to determine whether the measles was the cause of death, she added.
"Whatever the cause of death in this case, we should not be surprised if, as the outbreak grows, we start to see deaths in Wales," Lyons said.
If confirmed, it would be the first death from measles in Britain since 2008.
The cause of death will not be known until the results of the man's post-mortem examination are released.
Since November, 808 cases of measles have been confirmed in Wales, compared to just 116 in the whole of 2012.
The age group most affected is children aged 10 to 14.
The outbreak has been linked to the low uptake of the combined Measles Mumps and Rubella (MMR) vaccination, which is widely used across the world as the most effective prevention against measles.
Immunisations across Britain fell in the late 1990s following the publication of research suggesting the MMR was linked to an increased risk of autism and bowel disorders.
Andrew Wakefield's 1998 study caused public outcry, and although his claims have been comprehensively proven to be wrong, health officials have struggled to restore public confidence in the jab.
The outbreak of measles in Wales sparked queues outside clinics as parents rushed to get their children immunised, while special vaccination sessions are also taking place in schools.
The disease causes a distinctive red-brown rash on the skin, fever, cold-like symptoms and fatigue.
It can cause serious complications in around one in 15 cases, leading to deafness, brain damage and even death.
However, two doses of the MMR vaccine offer almost guaranteed protection against the disease. Vaccination levels in Britain are now at a high of around 88 percent, officials say.