Why is it so difficult to trace the origins of food poisoning outbreaks?

Why is it so difficult to trace the origins of food poisoning outbreaks?: As illustrated by the 2011 E. coli outbreak in Germany, any delay in identifying the source of food poisoning outbreaks can cost lives and cause considerable political and economical damage. Scientists have now shown that difficulties in finding the sources of contamination behind food poisoning cases are inevitable due to the increasing complexity of a global food traffic network where food products are constantly crossing country borders, generating a worldwide network.