Listeriosis is a disease caused by Listeria monocytogenes (L. monocytogenes). L. monocytogenes is bacteria that usually infects some determined inhabitants, especially high risk patients such as the elderly, immunosuppressed patients and pregnant women. However, it can also affect people who do not have these risk factors. L. monocytogenes is widespread in nature being part of the faecal flora of many mammals and it is a common foodborne source. It is acquired by humans primarily through consumption of contaminated food. Besides, between 1% and 10% of the population is a faecal carrier of L.monocytogenes. Listeriosis may occur sporadically or in outbreaks. Infection causes a spectrum of illness, ranging from febrile gastroenteritis to invasive disease, including bacteraemia, sepsis, and meningoencephalitis. This infection has a low incidence, although it is undeniably increasing, particularly due to the rise of population of over 60 years old or of under 60 years olds with a predisposing condition. The diagnosis is complicated because of its incubation period and the different clinical manifestations. Also listeriosis has a high mortality despite adequate and early treatment. The importance of bacteraemia for L. monocytogenes lies in the infrequency of this bacterium and the high mortality, even with appropriate antibiotic treatment.