Twice as many people in Denmark contracted listeriosis in 2009 compared with the previous year – and noone seems to know why.

The Copenhagen Post reports the latest figures from the National Food Institute at the Technical University of Denmark (DTU) show that Denmark has the highest number of cases of the rare and potentially lethal bacterial disease (presumably within the EU).

Jens Kirk Andersen, an expert from the National Food Institute said that it is often difficult to identify the source of listeria infections because it may take anything from two days to two months before symptoms appear.