Background 

• Pathogen : The causative agent is the bacterium Listeria monocytogenes. This bacterium is naturally prevalent in the environment, and it also occurs naturally in the intestines of many mammals, including humans.

• Transmission: Infection is almost always foodborne, meaning that it occurs as a result of the consumption of food containing L. monocytogenes. The bacterium can grow under cold conditions, so refrigeration does not prevent growth. Human‑to‑human transmission is very rare, except from mother to fetus/infant during pregnancy or childbirth. For more information, please see the dedicated page on the outbreak from the Swedish Health Agency (Folhälsomyndigheten).

• Sources and risk factors: Typical sources include ready‑to‑eat foods, refrigerated foods with long shelf‑life, smoked fish, soft cheeses, vacuum‑packed cold‑cuts, and other chilled foods where L. monocytogenes can multiply slowly. High risk groups for serious illness typically include the elderly, immunocompromised individuals, and pregnant people (with both mother and fetus at risk).

• Seasonality: There is no seasonality to infections. Food storage conditions (especially for chilled foods) are more important than seasonal weather patterns.

Brief Summary of the Outbreak

Since the end of September 2025, a number of people have fallen ill with confirmed L. monocytogenes infections after eating at the same restaurant in Stockholm. The regional public health authority (Smittskydd Stockholm) is investigating the outbreak, and Folkhälsomyndigheten is coordinating national‑level work, including the genetic analyses of isolates. Folkhälsomyndigheten has created a dedicated webpage with more information about their work on the outbreak.

Visits to the restaurant that resulted in infection occurred over a short period (23–27 September). A total of 15 confirmed cases (individuals with L. monocytogenes in their blood) were identified as of mid-October. Two of the isolates sequenced from these cases were genetically identical and not previously seen in the national database. Environmental/food source investigations are ongoing.

L. monocytogenes typically only causes serious illness in those in high risk groups. However, in this case, healthy people have became ill, with some having developed sepsis soon after exposure. The outbreak is therefore considered unusual for L. monocytogenes infections in Sweden.

Outbreak Timeline