The Centers for Disease Control and Prevention (CDC) and State Health Departments are investigating a cluster of Listeria infections. Thus far, Listeria bacteria isolated from 32 patients have been shown to have matching DNA patterns; this strongly suggests that these patients acquired illness from the same food. Case-patients live in southeastern Pennsylvania (14), New York City (11), New York State (3), Maryland (2), Connecticut (1), and Michigan (1). All patients were hospitalized and six persons have died (2 in New York City, 2 in Pennsylvania, 1 in Michigan and 1 in New York State). Federal and state public health officials are working to determine the food that caused these illnesses. These illnesses were first reported in the summer, and cases continued to be reported in September.

In addition to the 32 patients whose illnesses have been confirmed as part of the cluster, CDC and State Health Departments are investigating other cases of Listeria infection in the same region. Strains from some patients have already been shown to be different from the outbreak strain; these illnesses are part of the "background" of sporadic Listeria infections that are expected to occur. However, testing of Listeria isolates is ongoing; some of these will likely be the outbreak strain and some will not.