What are the symptoms of Listeria infection?
The infection caused by Listeria can vary from mild to severe
When a person is infected and develops symptoms of Listeria infection, the resulting illness is called listeriosis. [4, 11, 18] Only a small percentage of persons who ingest Listeria fall ill or develop symptoms.  For those who do develop symptoms as a result of their infection, the resulting illness is either mild or quite severe, in what is sometimes referred to as a “bimodal distribution of severity.” [13, 28]
On the mild end of the spectrum, listeriosis usually consists of the sudden onset of fever, chills, severe headache, vomiting, and other influenza-type symptoms. [18, 28] Along these same lines, the CDC notes that infected individuals may develop fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea.  When present, the diarrhea usually lasts 1-4 days (with 42 hours being average), with 12 bowel movements per day at its worst. 
Most healthy adults and children who consume contaminated food experience only mild to moderate symptoms. The infection is usually self-limited, since, in healthy hosts, exposure to Listeria stimulates the production of tumour necrosis factor and other cytokines, which activate monocytes and macrophages to eradicate the organism. Few people with normal immune function go on to have more severe, life-threatening forms of listeriosis, characterized by septic shock, meningitis and encephalitis. 
As already noted, when pregnant, women have a mildly impaired immune system that makes them susceptible to Listeria infection.  If infected, the illness appears as an acute fever, muscle pain, backache, and headache. [18, 24] Illness usually occurs in the third trimester, which is when immunity is at its lowest.  Infection during pregnancy can lead to premature labor, miscarriage, infection of the newborn, or even stillbirth. [24, 28] Twenty-two percent of such infections result in stillbirth or neonatal death. 
Newborns may present clinically with early-onset (less than 7 days) or late-onset forms of infection (7 or more days).  Those with the early-onset form are often diagnosed in the first 24 hours of life with sepsis (infection in the blood). [3, 18] Early-onset listeriosis is most often acquired through trans-placental transmission. [18, 24] Late-onset neonatal listeriosis is less common than the early-onset form. [4, 18, 24] Clinical symptoms may be subtle and include irritability, fever and poor feeding.  The mode of acquisition of late-onset listeriosis is poorly understood. [18, 24]