A virus that can kill great tits was brought into the UK by insects, scientists believe.
The disease was first found in south-east England in 2006, but has spread rapidly around the country. It causes large growths on birds' beaks and eyes.
A new study shows the strain originated in Scandinavia or Central Europe and was probably carried across the Channel by biting insects, such as mosquitoes.
The findings have been published in three papers in the journal Plos One.
"The lesions can be very severe," said Dr Becki Lawson, a veterinarian from the Zoological Society of London (ZSL).
"There is a very significant adverse impact on the individual bird's survival."
The virus is a form of avian pox, which is a skin disease.
Avian pox usually infects birds such as house sparrows, dunnocks and starlings.
But genetic analysis revealed that the strain hitting great tits in the UK is a different form of the virus, which originated in Europe.
Although the disease can pass from bird to bird, scientists do not think that great tits carried it into the country because the birds do not migrate across the English Channel.
Dr Lawson said: "It is more likely to be an insect vector, such as a mosquito, either moved by man or by wind-borne spread."
The study charted the movement of the disease across the country. Since it arrived, it has spread across central England, Wales and some cases are suspected in Scotland.
A bird-monitoring study that has been running for more than 50 years in Wytham Woods in Oxfordshire also helped to shed light on the emerging disease.
Researchers from the University of Oxford found that a range of tit species can catch the virus, but great tits were the most susceptible.
"Our results show that this new strain... significantly reduces the survival of wild great tits and has particularly large effects on the survival of juvenile birds," said Dr Shelly Lachish, from the Edward Grey Institute at the University of Oxford.
However, the team found that some birds were able to recover from an infection, and computer models suggest that although the disease would continue to spread it might not have too significant an effect on great tit numbers.
Mike Toms, head of garden ecology at the British Trust for Ornithology (BTO), who also worked on the study, said: "What we've not seen yet is an impact in the wider population level.
"Our models don't predict that its going to cause a population decline nationally - but we need to be aware of the possibility."
Because the disease can spread between birds and possibly through contaminated surfaces, he recommended that people should keep their feeding stations and feeders clean.
"As always, clean them on a regular basis: ideally every week or fortnight, by cleaning, rinsing and then air drying them," Mr Toms explained.
The scientists stressed that the virus could not pass from birds to humans.
The research team told BBC News that any sightings of birds displaying symptoms of the pox should be reported to the RSPB's Wildlife Enquiries Unit.
Dr Lawson said that the team of scientists, ornithologists and vets who carried out the research would not have been able to study the emerging disease without the help of the public.
"We can't do this kind of work, to find out about the health of British wildlife, unless the public take the time to keep their eyes peeled and and report signs of sick and dead birds to us," she said.
Avian pox is not the only new disease that is having an impact on garden birds.
In 2005, a disease called trichomonosis, which is caused by a parasite, was discovered in finches in the UK.
It has since spread throughout the country and has also been reported in the Republic of Ireland.
Greenfinches have been worst affected.
A recent paper published in a Royal Society journal revealed that about 1.5 million breeding birds have been lost, and the number of greenfinches visiting gardens has declined by about 50%.