Nitrosamines are cancer-causing compounds produced when meat preservatives called nitrites interact with compounds in meats called secondary amines in the presence of acid (like the stomach). High temperatures, as in frying, can also enhance the formation of nitrosamines.
Nitrosamines are found in many foods, especially beer, fish, and fish byproducts, and also in meat and cheese products preserved with nitrite pickling salt. The U.S. government established limits on the amount of nitrites used in meat products in order to decrease cancer risk in the population.
Jakszyn P, Gonzalez CA. Nitrosamine and related food intake and gastric and oesophageal cancer risk: a systematic review of the epidemiological evidence. World J Gastroenterol. 2006 Jul 21;12(27):4296-303.