Here's a recent FDA Listeria outbreak update via Businessweek:
"A dump truck parked near the open-air packing facility is among the possible causes of the contamination, Jim Gorny, a senior adviser for produce safety at the FDA, said on the conference call. The truck had hauled misshapen cantaloupes away from the facility and dumped them at a cattle ranch.
“Cattle are definitely a reservoir” of listeria and can transmit it through their feces, Gorny said. FDA inspectors later found the truck “parked in the general proximity, right next to where food is handled,” he said."