Infectious dose/dose response: Epidemiological evidence from investigations where the vehicle of infection has been identified indicates that foods contaminated with less 100 cfu/g of L. monocytogenes are unlikely to cause illness in the general population. There is one study that suggests that the level of L. monocytogenes needed to cause illness in susceptible groups may be lower (Maijala et al. 2001).
Factors affecting the likelihood of illness developing in an individual consumer may include their immune status, the type of food consumed, the virulence and infectivity of the pathogen, the concentration of the pathogen in the food, and the number of repetitive challenges (National Advisory Committee on Microbiological Criteria for Foods 1991). Thus, even when an outbreak occurs not all people consuming the contaminated foods will develop an infection.
