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There's usually a laundry list of dos and don'ts that people can expect to hear when they're pregnant. Don't eat fish, don't drink caffeinated beverages— and they don't always tell you why.

One of the most oddly specific-sounding ones is a moratorium on soft cheeses. And there's actually an extremely good reason for it, although the focus shouldn't necessarily fall on Camembert. A number of processed foods, including but not limited to soft cheeses, can harbor the infectious bacterium Listeria—and it's a serious danger to fetuses.

Listeria monocytogenes is a common bacterium found in a huge variety of places, including water, soil, raw produce, processed foods—and even feces. It's actually a pretty uncommon infection in the general population, but one exception is pregnant people, for whom it's nearly 20 times more common. A lot of changes happen to your body during pregnancy.

This includes your immune system, specifically a reduction in cell-mediated immunity. This system includes multiple defense mechanisms that don't rely on antibodies. Instead it's mediated by specialized cells.

This type of immunity is important for detecting and destroying cells in your body that may be infected with viruses or bacteria. Your antibody-based immunity, on the other hand, makes those antibodies in response to viruses or bacteria in the fluid outside of your cells. But your body shifts away from cell-mediated immunity during pregnancy to allow a tiny human to grow inside of you.

This ensures those well-meaning immune cells don't detect the fetus and treat it like an unwanted intruder. Although Listeria infection is far more common in pregnant people, symptoms tend to be very mild. This is because Listeria is a unique bacterium with a sneaky way of getting around that doesn't get your antibody-based immune system all riled up.

The bacteria basically trick the cells in your gastrointestinal tract into engulfing them. Once inside a cell, Listeria multiplies and pushes against the membrane, eventually pushing straight through into neighboring cells without spending any time in the outside space. This means your best defense against Listeria is your cell-mediated immunity.

So it makes sense that there's less of a reaction in a pregnant host. But this sneaky way of spreading also means it can get across the placenta, and this is where it becomes really risky. The human maternal-fetal interface is what nourishes your fetus and protects it from both your immune cells and outside infections.

Placental cells have direct contact with maternal blood, so the placenta is very resistant to infections, but not to pathogens like Listeria. And researchers aren't totally sure why yet. Although Listeria doesn't usually have much of an impact on the pregnant person, it's very dangerous to developing fetuses.

It can cause premature birth or pregnancy loss as well as brain damage, and is often fatal to fetuses and newborns. We mostly get Listeria infections from contaminated foods, and we can take some steps to limit our exposure to it. Like pasteurization, which eliminates Listeria from dairy products.

So although you can get it from a variety of things, unpasteurized cheese is definitely on the no-go list. In some parts of the world, including the US, it's actually hard to find unpasteurized cheese. But Listeria has also been found in pasteurized soft cheeses thanks to manufacturing issues.

Soft cheese gets a lot of attention when it comes to Listeria risk, but there's actually a wide range of dairy products, cold meats, and other processed foods that are vulnerable to contamination. Recent outbreaks of Listeria have even been linked to unprocessed foods, like raw sprouts, celery, and melon. Like we said—Listeria is super common, and it's also behind the prohibition of stuff like hot dogs and deli meats for pregnant people.

So to be on the safe side, it might be a good idea to just avoid the deli counter for the duration of pregnancy! Thanks for asking, and thanks especially to our patrons for supporting this episode. Patrons have a chance at getting their questions answered in episodes like this one.

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