Snopes, the keeper of all Urban Legends, has researched the legend that shopping carts (known to many in the South as "buggies") are exceptionally filthy. Snopes' conclusion? Yes they are:
The very folks that fuss with disposable toilet seat covers often won't think twice about manhandling a grocery cart around a store for half an hour, even though the surface they're hanging onto so fiercely for that interval has likely been coughed on, sneezed on, and grasped by shoppers who'd just finished handling packages of raw chicken and meat, or who just weren't the sort to wash their hands all that often. . . .
According to a four-year study conducted by the University of Arizona's Environmental Research Lab and sponsored by Clorox, grocery carts are veritable petri dishes teeming with human saliva, mucus, urine, fecal matter, as well as the blood and juices from raw meat. Swabs taken from the handles and child seats of 36 grocery carts in San Francisco, Chicago, Tucson, and Tampa showed these common surfaces to rank third on the list of nastiest public items to touch, with only playground equipment and the armrests on public transportation producing more disgusting results.