Listeria, or Listeria monocytogenes to be exact, is a bacteria that might appear in milk and milk products. BUT it might just as well appear in may other products as well without any connection to milk. Chicken and eggs are well known sources. Even water and earth.
Cheese, along with other foods consumed without any heat treatment are regarded more risky than other foods. Some think that pasteurized cheese is 100 % safe, but that is not true. Firm and hard cheeses are safe, irrespective of being from raw or pasteurized milk.
The USA, fearing most things, have normally one serious outbreak of listeria stemming from cheese every year or so. They have made it illegal to sell raw milk cheese younger than 60 days. 60 days is as long as a listeria bacteria can survive, according to scientists in the USA. That is a topic which is being discussed. Not everyone agrees to that. However, most of the outbreaks in recent years have come from pasteurized cheese, with 2017 being an exception. The last big European outbreak of listeria from cheese came from Epoisses that was pasteurized and the last general outbreak in Europe came from beansprouts.
Listeria does not always appear harmful if you are a healthy person. Most of us will think we have a light flu. If you are old, have reduced immune system or you are pregnant, listeria may represent a serious risk. For pregnant women the case might be that the mother thinks she has a light flu, while the fetus will be seriously harmed.
Pasteurization kills the harmful bacteria, if any. If the milk or a cheese, however, is exposed to listeria at some stage after pasteurisation, it will be contaminated.
Regarding cheese and listeria there are a few factors to remember: Soft cheese are connected with a certain risk, firm and hard cheeses are regarded a safe. The reason firm and hard cheeses are regarded as safe is there is not enough “food” for the bacteria to grow. Soft cheeses are cheese like any soft cheeses, bloomy rind, washed rind and blue. In these types of cheese there is a lot of nutrition for the listeria bacteria to grow and thrive. So, the more dry matter the cheese has, the safer it is. Longer maturation is is better than short. These factors applies to cheese from both raw and pasteurised milk.
The general rule is to be aware of what you are eating, and especially if you are pregnant. And if you are in doubt, don’t do it.