A mysterious listeria outbreak across the United States has left health officials trying to find the reasons for the outbreak.

So far, no specific food product has been identified as the source of the outbreak. The Centers for Disease Control and Prevention (CDC) has received several recent reports of illness, giving cause for concern.

Listeria monocytogenes is a species of pathogenic bacteria that can bring about the serious infection of listeriosis. Symptoms are often similar to those of common food poisoning.

According to the CDC, approximately 1,600 people get infected with the bacterium each year. High-risk groups include pregnant women and newborns, as well as elderly adults and those who suffer from weakened immune systems or auto-immune disorders.

A condition called invasive listeriosis occurs when the bacteria disseminate past the gut to other areas of the body. People usually experience symptoms within two weeks after eating food contaminated with the pathogen.

During pregnancy, infection may lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn, while in almost 5 percent of cases for other groups, the infection can prove fatal.

Common symptoms in those who are not pregnant include fever and flu-like symptoms such as headaches, aches and muscle fatigue, as well as stiff neck, loss of coordination and balance, and seizures.

Currently, outbreaks have been observed throughout the country, with Michigan having the highest number of reported cases.

To date, 10 people with the symptoms have been hospitalized. The ages of those affected range from 47 to 88 years, with a distinctive majority being female, according to an article published by The Hill.

According to the CDC, in some cases, the onset of symptoms can occur earlier than two weeks after eating contaminated food, even on the same day—or as late as 10 weeks after. The most recent outbreak had reported illnesses as far back as July 2018, The Hill reported.