For otherwise healthy people, Listeria usually presents as a “stomach bug.” Mild infections are rarely diagnosed, because people may not even require medical care. In these circumstances, the incubation period is shorter: 24 hours, on average.
However, in people that are young, elderly, pregnant, or otherwise immunocompromised, a Listeria infection can be far more dangerous. Listeria can infect the nervous system (Listeria meningitis), the blood (bacteremia), joints, and, for pregnant women, cause harm to the developing baby.
These are hallmark symptoms of meningitis. This type of Listeria infection has an average incubation period of 9 days, but it can vary from 1-14 days.
Pregnant women are at a greater risk for Listeria infections because their immune systems are weaker. Expectant moms are 20 times more likely to be infected with Listeria than the general population. However, pregnant women may not notice many symptoms themselves—potentially mild gastrointestinal symptoms—but the Listeria infection can be devastating for the baby. In early pregnancy, Listeria is known to cause miscarriage. Later in pregnancy, Listeria can cause preterm labor and even fetal death. Newborns infected with the disease may show signs of difficult breathing, fever, rash, jaundice, a poor appetite, and excessive sleepiness. For pregnant women, the incubation period is often longer, ranging from 17-67 days.