Listeriosis is an infection caused by eating food contaminated with the Listeria monocytogenes bacterium.
The bacterium can also be found in soil and water.
Dr Juno Thomas, of the National Institute of Communicable Diseases, said she was concerned that the disease would spread during the festive season when people would have large family meals together.
Food might also not be stored at the right temperatures on hot days, which worked in favour of the bacterium.
The disease has already claimed 36 lives in South Africa and there are more than 550 others infected.
Thomas did not know how many people had died in KwaZulu-Natal because the institute was still collecting information.
She said the disease originally broke out in Gauteng and the Western Province, and there were later cases in KZN.
Environmental health practitioners were tracking the disease by taking food samples from sick people.
Thomas said they had been receiving assistance from the Department of Health and the private sector in KZN to track where the disease had come from.
Health Minister Aaron Motsoaledi said they believed the disease had originated at a farm or food processing plant.
He said there were normally 60-80 cases of the disease every year, but that doctors had noticed a sharp spike in the number of infections this year.
“Tracing from January 1 to November 29, a total of 557 laboratory-confirmed Listeriosis cases have been reported from all provinces,” Motsoaledi said.
Of these cases, 34% were from private health institutions.
“Given that only 17% of South Africans use private health facilities, this proportion of cases from private health facilities is too high.
“This indicates that the source of the outbreak is likely to be a food product that is widely distributed and consumed by people across all socio-economic groups,” he said.
KZN is third on the list with 37 cases reported. Gauteng confirmed 345 and the Western Province, 71.
The three provinces were responsible for more than 80% of all cases, Motsoaledi said.
It was concerning that a large number of people who were infected went to private health facilities.
“Although anyone can get Listeriosis, those at high risk of developing the severe disease include newborns, the elderly, pregnant women and people with weak immunity such as HIV, diabetes, cancer, chronic liver or kidney disease patients,” Motsoaledi said.
When people were infected, they would normally display a “flu-like illness with diarrhoea, fever, general body pains, vomiting and weakness”, among other symptoms.
Motsoaledi cited the World Health Organisation’s food safety tips to prevent possible infection.
These include cooking food thoroughly, particularly meat products, washing hands thoroughly before and after preparing meals, and making sure that clean water is used.
If families have no clean water supply, they should boil the water.