A listeria outbreak has left 57 sick, 9 dead. Here's what we know so far

Dozens of Americans have been sickened and at least nine people have died in a listeria outbreak linked to Boar's Head deli meat.

Officials from the Centers for Disease Control and Prevention (CDC), the U.S. Department of Agriculture (USDA) and other public health or regulatory agencies have been investigating the multi-state outbreak, which began in May.

Recently, documents from the USDA Food Safety and Inspection Service (FSIS) showed that the Jarratt, Virginia meat plant linked to the deadly outbreak had 69 recorded instances of noncompliance over the last 12 months, including mold, insects, blood puddles on the floor, discolored meat and fat buildup on the walls, floors and equipment, and a "rancid smell" coming from a cooler.

The FSIS documents do not mention listeria, but the bacteria can grow in areas that are not properly cleaned and can remain active on surfaces even at refrigerated temperatures.

Here's everything we know about the outbreak so far:

Who has been sickened?

As of Aug. 28, there have been 57 people who have fallen ill against across 18 states. All 57 people have been hospitalized and nine people have died, according to the CDC .

States with cases include Arizona, Florida, Georgia, Illinois, Indiana, Maryland, Massachusetts, Minnesota, Missouri, New Jersey, New Mexico, New York, North Carolina, Pennsylvania, South Carolina, Tennessee, Virginia, and Wisconsin.

New York has the most reported cases, with 17, followed by Maryland with eight cases and New Jersey with five cases.

The first case linked to the outbreak was reported on May 29 and the most recent case was reported on Aug. 16, according to CDC data . However, more recent illnesses may not yet be reported because it takes three to four weeks, on average, to determine if an illness Is linked to an outbreak, according to the federal health agency.

Additionally, the true number of sicknesses linked to the outbreak is likely higher than reported because some people recover without medical care and are not tested for listeria, the CDC said.

What's behind the outbreak?

The outbreak has been traced back to Boar's Head brand meats sliced at delis, as well as ready-to-eat liverwurst, that were contaminated with listeria, according to epidemiologic, laboratory and traceback data reported by the CDC.

The problem was discovered when the FSIS received notice of a liverwurst sample collected by the Maryland Department of Health that was positive for Listeria monocytogenes (L. monocytogenes), the bacterium that causes listeria infection.

The Boar's Head ready-to-eat liverwurst products were produced between June 11 and July 17 and had a 44-day shelf-life, with sell-by dates between July 25 and Aug. 30.

The Boar's Head plant linked to the outbreak also recalled more than seven million pounds of deli meat and poultry products – including meat intended for slicing at delis and pre-packaged products – produced between May 10 and July 29, according to a notice posted on the USDA FSIS website.

Recalled products contain "EST. 12612" or "P-12612" inside the USDA mark of inspection on the product labels, according to the notice.

"We deeply regret the impact this recall has had on affected families," Boar's Head said in a statement on Thursday. "No words can fully express our sympathies and the sincere and deep hurt we feel for those who have suffered losses or endured illness."

The company said production has paused at the Jarratt, Virginia facility and will not resume until it meets USDA regulatory standards.

What to know about listeria infections

Consuming food contaminated with the bacterium L. monocytogenes can cause a serious infection known as listeriosis . The infection can progress to invasive listeriosis when the bacteria spread beyond the gut to other parts of the body, according to the CDC.

Symptoms of severe illness usually begin one to four weeks after eating the contaminated food but can present as early as the same day, or as late as 70 days later, the CDC said.

An estimated 1,600 people get listeriosis annually in the U.S. and about 260 people die from it, according to the CDC. Those most at risk include pregnant people and newborns, adults aged 65 or older, and people with weakened immune systems.

Those who are not pregnant are likely to experience symptoms including fever, headache, muscle aches, fatigue, stiff neck, confusion or loss of balance, the CDC said, while those who are pregnant are likely to experience just a fever, fatigue and flu-like symptoms. However, listeria infection during pregnancy can increase the risk of miscarriage, still birth, and premature delivery.

The CDC recommends those at higher risk of contracting listeriosis should avoid eating meat sliced at deli counters unless it's first heated to an internal temperature of 165 degrees Fahrenheit, or until it's steaming hot just before serving. The agency also recommends calling your health care provider if you have symptoms of listeriosis and you recently consumed meat sliced at a deli counter.