Transmission of and Infection with Listeria
Listeria typically spreads to people through contaminated food or water, but can also be transmitted from mother to fetus.
Except for the transmission of mother to fetus, human-to-human transmission of Listeria is not known to occur.  Infection is caused almost exclusively by the ingestion of the bacteria, most often through the consumption of contaminated food. [18, 21, 23] The most widely-accepted estimate of foodborne transmission is 85-95% of all Listeria cases. [23, 28]
The infective dose—that is, the amount of bacteria that must be ingested to cause illness—is not known. [4, 18, 26] In an otherwise healthy person, an extremely large number of Listeria bacteria must be ingested to cause illness—estimated to be somewhere between 10–100 million viable bacteria (or colony forming units “CFU”) in healthy individuals, and only 0.1–10 million CFU in people at high risk of infection. [4, 18, 26] Even with such a dose, a healthy individual will suffer only a fever, diarrhea, and related gastrointestinal symptoms. [4, 18].
The amount of time from infection to the onset of symptoms—typically referred to as the incubation period—can vary to a significant degree. Symptoms of Listeria infection can develop at any time from 2 to 70 days after eating contaminated food. [4, 5] According to one authoritative text:
The incubation period for invasive illness is not well established, but evidence from a few cases related to specific ingestions points to 11 to 70 days, with a mean of 31 days. In one report, two pregnant women whose only common exposure was attendance at a party developed Listeria bacteremia with the same uncommon enzyme type; incubation periods for illness were 19 and 23 days. 
Adults can get listeriosis by eating food contaminated with Listeria, but babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. [4, 24] The mode of transmission of Listeria to the fetus is either transplacental via the maternal blood stream or ascending from a colonized genital tract.  Infections during pregnancy can cause premature delivery, miscarriage, stillbirth, or serious health problems for the newborn. [18, 24]
Incidence of Listeria infection in HIV-positive individuals is higher than in the general population. [17, 18] One study found that:
The estimated incidence of listeriosis among HIV-infected patients in metropolitan Atlanta was 52 cases per 100,000 patients per year, and among patients with AIDS it was 115 cases per 100,000 patients per year, rates 65–145 times higher than those among the general population. HIV-associated cases occurred in adults who were 29–62 years of age and in postnatal infants who were 2 and 6 months of age. 
Pregnant women make up around 30% of all infection cases, while accounting for 60% of cases involving the 10- to 40-year age group.