South Africa health officials say they have finally traced the sources of a listeria outbreak that has infected 940 people and killed 180 over the past year, making it the worst currently and probably one of the worst in history.

The main source of the listeriosis outbreak has been traced to the Enterprise Food facility in Polokwane, which produces Polony - the local version of Devon sausage, or Bologna.  The factory will likely be shut down, while two other food production units remain under investigation.

"We advise members of the public to avoid all processed meat products that are sold as ready-to-eat," said Health Minister Aaron Motsoaledi, warning that pregnant women should avoid it "like the plague".  Consumers who bought infected products are being advised to wash out their refrigerators with bleach to kill the germs.

This is a particularly strain called Listeriosis monocytogenes ST 6.  It's difficult for health inspectors to detect because it is not homogenously distributed in food.  It can also "hide away" in cracks or niches in factories. 

Listeria can infect meat, smoked seafood, raw milk, cheese, veggies like sprouts, and - as Australia recently found out - melons.