As Live Science notes, it’s possible for Listeria bacteria to be in ice cream. Benjamin Chapman, a food safety expert at North Carolina State University in Raleigh, told Live Science that usually for Listeria to cause sickness, there would have to be ” … tens of thousands, or hundreds of thousands of cells … ” of it in a single serving of a food. But as Amreen Bashir, lecturer in Biomedical Science at Aston University, explains via The Conversation, the milk and sugar in melted ice cream can create favorable conditions for bacteria (including Listeria) and recommends not letting ice cream thaw and then refreezing it.
A Listeria infection can range from a mild intestinal illness to a potentially deadly invasive infection, via CDC. Although anyone can become ill from Listeria, the odds go up for newborns, as well as individuals who are 65 years old or older, are pregnant, or have weakened immune systems (per CDC). A Listeria invasive illness in someone who is pregnant can result in a premature delivery, a stillbirth, a miscarriage, or the newborn having a potentially deadly infection.
Listeria as an intestinal illness can involve vomiting and diarrhea (via CDC). Listeria as an invasive illness can cause flu-like symptoms and a fever in both pregnant and non-pregnant individuals, and someone who isn’t pregnant might also experience balance issues, headaches, confusion, seizures, and neck stiffness. So, don’t eat ice cream if you suspect it contains even a small amount of Listeria.