In summary, little is known about why certain people are more susceptible to infection by some of the frequent colonizers of the human nasopharynx. Interestingly, opportunistic pathogens such as Nm and Hi as well as M. catarrhalis (not investigated here) target CEACAM1. As specific substitutions such as Y > F and Q > N produce more favourable targets for distinct mucosal isolates, it is possible that occurrence of such receptor polymorphisms in the human population could lead to greater bacterial binding thus increasing the chances of cellular invasion. Given the colonization rate of these organisms (generally > 10% of the population) and the frequency of invasive infection (up to 3:100 000 population), a combination of events may be required to increase host susceptibility. Inflammatory conditions that increase receptor density in populations carrying specific polymorphisms could provide the worst scenario.