Thus, more realistic models should take into account that adaptive immunity systems are characterized by the existence of immune memory that enhances the response in individuals encountering a familiar challenge [ 37, 38].
Although adaptive immunity is a self-protection mechanism, its protective effects often extend well beyond the individual [ 22], since the existence of such immunity greatly influences the transmission dynamics of the pathogen in the population as a whole.
The reasons for this low mortality among elderly may relate to the existence of prior immunity of individuals born before 1957 after infection with an A(H1N1 2009 antigenically related virus [ 15, 16].
However the existence of natural immunity to infection in some humans (Mehta et al., 2002) and chimpanzees (Bassett et al., 2001; Farci et al., 1992; Lanford et al., 2004; Weiner et al., 2001) is encouraging and suggests that the immune system can eliminate infection.
We can not exclude the existence of low-immunity pockets in the population that were not detected in our study.
Since no information is available on the existence of cross-immunity from past flu infections, the initial proportion of susceptible individuals was set to 100%.
However, it is difficult to provide a more accurate prediction since no information is available on the existence of cross-immunity from past flu infections.
We and others have previously demonstrated the existence of anti-mycobacterial immunity induced by environmental mycobacteria in BCG naïve adolescents and adults in the UK [14], [18].
Malignant mesothelioma is caused by exposure to asbestos, but its development is limited by the parts that have been exposed to asbestos, suggesting the existence of effective antitumor immunity against transformed cells at an initial phase in the body of individuals exposed to asbestos.
Existence of cross-reactive immunity between previous influenza infections and the 2009 pandemic influenza has been observed in several reports, and demonstrated that the preexposure to 1918 pandemic influenza H1N1 or to 2007 seasonal influenza H1N1 might have enhanced the cross-reactive neutralizing antibodies against the 2009 pandemic influenza [ 9, 10].
