The head of the German public health body tackling a deadly E.coli outbreak says it may be months before it stops.Reinhard Burger, president of the Robert Koch Institute, told the BBC "we may never know" the infections' source.
He expressed sympathy for Spanish farmers affected by the false charge that their cucumbers were to blame.
More than 1,500 people have been infected by enterohaemorrhagic E.coli (EHEC), which can cause the deadly haemolytic-uraemic syndrome (HUS).
Seventeen people have died - 16 of them in Germany and one in Sweden.
Meanwhile, Russia has banned the import of fresh vegetables from European Union countries. A quarter of all vegetables exported from the EU are sent to Russia.
Russian consumer protection agency head Gennady Onishchenko announced the ban and criticised health standards within the EU.
"This shows that Europe's lauded health legislation - one which Russia is being urged to adopt - does not work," he said.
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