Relationship with Humans

Listeria is found mostly in nature; in places such as soil. What causes human relations is when the pathogen form of listeria is food borne and infects humans.

Listeria monocytogenes is the only form of listeria that causes listeriosis out of all 8 types of the bacteria. It has the ability to survive extreme cold and heat, so it doesn't die through food processing. When humans or animals consume bacteria infected foods, they may develop the listeriosis illness. Once the listeria bacteria leaves the digestive system and goes into the blood stream, the host is then infected with listeriosis and begins to show symptoms.

The listeria bacteria can be contracted through eating or touching soft dairy products (like cheese), vegetables, fish, or meats that are contaminated and were not prepared properly. It has the highest death rate (25%) of all food borne illnesses.

Types of food that listeria may contaminate
From http://healthwise-everythinghealth.blogspot.ca/2011/09/what-is-listeria.html

In many healthy children and adults (under age 60) the bacteria will be eliminated before it causes any symptoms or show very mild and non life threatening symptoms. In fact, 5-10% of humans and animals are found to have the bacteria in their digestive system, and show no symptoms.

In other cases, the bacteria will infect people who have weakened immune systems, such as pregnant women, ill, or the elderly. In this case, listeriosis can be life threatening causing symptoms such as: meningitis, nausea, cramps, diarrhea, brain/blood infections, and other flu like symptoms. During pregnancy, the listeria bacteria are able to pass through the placental barrier and infect the fetus causing stillbirth or miscarriages.