The endangered red squirrel population of Anglesey's Newborough Forest is facing a new killer virus, experts say.
Other red squirrels will have been exposed to the deadly virus
One animal is known to have died of a deadly adenovirus, the first documented case in Wales. Two others are thought likely to be diagnosed with the virus.
Conservationists say it is a "worrying development" in work to protect the island's squirrel population of 200.
Earlier this year, the reds faced an outbreak of "squirrel pox", carried by grey squirrels, but it was averted.
Scientists hope to use the same techniques - washing the nest boxes with an antiviral disinfectant - to head off the spread of the latest threat, which causes a fatal diarrhoea in infected animals.
Dr Craig Shuttleworth, woodland ecologist at Menter Môn, which runs the Anglesey red squirrel project, said seven cases of red squirrel adenovirus have previously been recorded in Britain, all in north west England.
An outbreak of squirrel pox has already been averted, experts say
He said the symptoms - typically unusual lesions and diarrhoea - were hard to confirm as the squirrels' intestines began to decompose within three hours and their bodies were unlikely to be found in time for tests to work.
The animal which tested positive had one of the highest viral counts ever observed in a red squirrel, said Dr Shuttleworth, and it was likely that other wild red squirrels would have been exposed to the virus.
He said scientists were becoming increasingly concerned about the possibility that the virus may have caused severe declines in red squirrel populations in previous years.
"We have been checking all of our red squirrel nest boxes for dead animals, and have cleaned all of our red squirrel feeders with an anti-viral wash," said Dr Shuttleworth.
"At the moment we have no evidence of an island-wide disease epidemic."
The three squirrels were found over a seven-day period in September.
The test results on the first confirmed case were passed on to Menter Môn by the Vetinarary Laboratories Agency in Surrey last Friday.
Anglesey's red squirrel population is shared between Pentraeth and Newborough forests and the broadleaved woodlands around Beaumaris.
In May, conservationists celebrated after the steps to defeat squirrel pox, including removing all grey squirrels, prevented an outbreak of the disease in 700-hectare Newborough Forest.