A measles epidemic is believed to have claimed its first victim with health experts warning the public to expect more.
The person, named locally as Gareth Williams, 25, of Port Tennant, Swansea, South Wales, reportedly died at home on Thursday morning.
The coroner for Swansea, the centre of the epidemic, has ordered an examination of his body to establish whether measles, or something else, is the cause. A spokesman for the corner's office confirmed that it is investigating the "unexplained" death of a 25-year-old man.
Public Health Wales (PHW) has also confirmed that it is aware that the city's coroner is investigating a death. But confirmation of the cause will not be known until the results of a post-mortem examination are released, possibly not until next week.
Neighbours on the Swansea estate where Mr Williams lived in a flat learned of the potential measles link to his death earlier.
"I think people were under the impression that he died from an asthma attack, that is what I heard," said one female neighbour, who did not wish to be named. "He was somebody you rarely saw, to be honest. He often used the back way into his flat so you would not even see him coming or going. But it was only when the police and TV cameras turned up and it was reported on the news that anybody heard anything about measles."
The death comes as the rate with which the outbreak is spreading continues to grow rapidly with 43 cases in the two days to Thursday. That brought the headline total, in what is now one of the biggest measles epidemics to hit the UK in a decade, to 808 cases.
Dr Marion Lyons, director of Health Protection at PHW, said: "Public Health Wales is aware of a death being investigated by the Swansea coroner. A diagnosis of measles has not yet been confirmed. Testing the deceased for measles is forming part of the investigations being overseen by the coroner."
She added: "My sympathies are with the family at such a tragic time. 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. Measles is a potentially fatal disease and around one in every 1,000 people who contracts measles in developed countries will die."
A total of 77 people have so far been taken to hospital as a result of the measles epidemic in South Wales. The majority of those needing treatment are believed to be babies and teenagers who had not previously been vaccinated.