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.
Spread to US
At least 365 new E.coli cases were reported on Wednesday, a quarter of them involving HUS, a condition associated with bloody diarrhoea and kidney failure, the Robert Koch Institute said.
The new cases include two in the US, both of whom had recently travelled to Hamburg, where many of the cases are clustered.
Professor Burger said it was impossible to say how long before the last case would appear:
"The number [of cases] will come down but how long it will take I am not sure. It could be weeks, months," he told the BBC.
The length of time would depend on whether infected food was still in warehouses, and whether the original source was still active, he said.
Spain has threatened to file a suit on behalf of its farmers against German authorities who had alleged a link between Spanish vegetables and the deadly strain of bacteria.
The European Commission lifted its warning over the Spanish cucumbers on Wednesday, saying tests "did not confirm the presence of the specific serotype (O104), which is responsible for the outbreak affecting humans."
The Spanish health ministry welcomed the move as "a very important step to restore normalcy as soon as possible to the Spanish agricultural sector".
The loss of earnings for affected farmers in Spain has been estimated at more than 200 million euros ($290 million) per week.
Mr Burger said German authorities had tried to balance risks when they wrongly blamed Spanish farms.
He said the authorities had to act quickly - even though the conclusion later turned out to be wrong.
"We wanted to avoid new infection sources. It's a difficult balance," he told the BBC.
"You don't want to wait a long time and on the other hand you don't want to cry wolf."
Asked what he would say to a farmer who had suffered a loss of trade, Mr Burger replied:
"I would feel very sorry and could totally understand his complaints. This was a poor, poor situation."
The BBC's Stephen Evans, in Berlin, said Mr Burger was frank in saying that health officials simply didn't know where the particularly virulent strain of of E.coli came from.
The source could be anywhere between any farm that grew cucumbers and tomatoes and a market in Hamburg, our correspondent adds.
In addition to Germany, cases of EHEC have also been reported in eight other European countries - Austria, Denmark, the Netherlands, Norway, Spain, Sweden, Switzerland and the UK.
Virtually all the sick people either live in Germany or recently travelled there.
Several countries have taken steps to curtail the outbreak, such as banning cucumber imports and removing the vegetables from sale.
Health authorities have also advised people to wash fruit and vegetables thoroughly, to do the same with all cutlery and plates, and to wash their hands before meals.