University of NSW Associate Professor Julian Cox said the best food and health authorities might come up with was identifying the "most likely" source.

Investigators would be hampered by a complicated chain of contamination, coupled with measures such as industrial cleaning conducted at affected aged care facilities to protect residents.

"Salmonella is a bacterium that has for a long time been associated with food poisoning," Prof Cox said. "While it originates from a host, the problem is that many types of salmonella are not limited to that particular type of host and can circulate in a complex way through the food chain.

"So while the classic culprits have been flesh foods like meat [and] chicken and raw foods like eggs, in more recent times there have been outbreaks involving fresh produce - fruit and veg.

"And some of the more obscure salmonella outbreaks have been linked to products like toasted muesli [in the US] and orange juice [in South Australia], with contaminated water present in both cases."

Though the salmonella bovismorbificans strain present in the Illawarra cases was originally associated with cattle, that did not only point to meat as a source, Prof Cox said.

Indeed the most recent Australian outbreak of this rare strain of salmonella in Queensland in 2001 was eventually linked to shredded iceberg lettuce.

"This type of salmonella continues to be associated with animals from which flesh foods are obtained - this includes cattle and sheep," he said. "But it could also exist in a range of hosts which find themselves in a whole range of foods, making it hard to find the smoking gun. The source might never be found."

Prof Cox said salmonella poisoning was of most concern in "YOPIs" - the young, old, pregnant and immuno-compromised.

"These days it is a fairly self-limiting gastro-intestinal disease causing diarrhoea, nausea and vomiting. An average healthy individual may be able to overcome that illness without any significant medical intervention just by staying well hydrated.

"When it becomes a real problem is in more vulnerable parts of the population. That's why it's such a concern when we have outbreaks in places like aged care facilities, as the aged generally have a weaker immune system and are less able to fight infection."