Public health agency THL has reported a rapid rise in the number of cases of Pogosta disease — a mosquito-borne viral infection with flu-like symptoms that is unique to Finland — during autumn this year.
In the middle of last month, the agency recorded 151 infections but this number had jumped significantly, to 467, by mid-October.
Some 83 cases have been found in blood tests carried out in Eastern Finland's North Savo region, which is the highest number of regional infections detected so far.
There have also been 65 cases registered in Central Finland, 64 in North Ostrobothnia and 56 in the Pirkanmaa region.
The last time such high numbers of the virus were detected nationwide was nearly 20 years ago, when 597 infections were recorded.
"This is now clearly a 'Pogosta year'," professor of virology at the University of Helsinki Olli Vapalahti told Yle.
The disease tends to be cyclical, meaning cases remain very low for about seven years and then there are suddenly hundreds of cases in one year.
The highest number of infections ever recorded in Finland was in 1995, when more than 1,300 cases were diagnosed.
Pogosta disease is caused by a virus that is transmitted through a mosquito bite, and the symptoms usually include a rash, mild fever and other flu-like effects — but in more extreme cases can lead to a painful arthritis.
The disease was first detected in 1974 in the North Karelian village of Ilomantsi, located on Finland's eastern border with Russia, and the word 'pogosta' derives from the Russian term for the area around the village church.