South African officials say an outbreak of listeriosis has killed 172 people since the start of 2017. It is the world’s largest reported Listeria outbreak.

The National Institute of Communicable Diseases said 915 cases of listeriosis had been confirmed since Jan. 1, 2017. Early last month, only 61 deaths had been confirmed.

In this image from the Pasteur Institute, Listeria monocytogenes (shown in red) is in the process of infecting tissue cells.

Listeriosis is caused by a natural bacteria found in soil, water, vegetation and animal feces and can contaminate food, especially meat, dairy, seafood and fresh produce.

It is prevented by basic hygiene and washing food, but South African authorities say the source of the worsening outbreak is still unknown.

The United Nations has said South Africa’s listeriosis outbreak is believed to be the largest-ever worldwide.

The disease has an incubation period of up to 70 days, making it difficult to traceback to specific sources.

Most of the current cases have been in Gauteng province, which includes the cities of Johannesburg and Pretoria, with other cases concentrated in Western Cape and KwaZulu-Natal provinces.

Contamination in humans can result in flu-like illness, infection of the bloodstream, and, in severe cases, infection of the brain — which can prove fatal.

People with compromised immune systems, like some of those living with AIDS and pregnant women, are at a heightened risk, according to the World Health Organization.

The WHO advises people to wash hands and kitchen surfaces, keep raw and cooked food separate, cook food thoroughly and store it in a refrigerator.

The National Institute of Communicable Diseases said it is optimistic the source of the outbreak will be found and urged members of the public not to panic.

It warned vulnerable people to avoid processed, ready-to-eat meat products such as ham and sausages, soft cheeses and unpasteurized milk and dairy products.

South African authorities said specialized tests are being conducted by experts at the National Institute’s laboratories to detect the source and end the outbreak.

The source of the outbreak is thought to be a food product or range of products from one company, but that is still just a theory. Victims include people from various social and economic backgrounds.

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