There are at least 90 different serotypes of Pneumococcus, which strongly differ in their prevalence, incidence and virulence. As so far little is known about the interaction between different serotypes, it is difficult to predict the effects of a wide-spread use of the newly developed vaccines. Because of cross-immunity, vaccination may also effect the transmission of serotypes which are not included in the vaccine. Wide-spread vaccination may lead to a decline in all serotypes covered by the vaccine or vulnerable to cross-reacting antibodies, but this may also lead to undesired side effects. Some other serotypes may be rare now because they are constantly outcompeted by the more prevalent types. If that is the case, any reduction in the prevalence of one serotype may completely change the frequencies of all other serotypes. It is intrinsically different to determine whether rare serotypes are more or less pathogenic than the more prevalent ones. In the worst possible case, wide-spread vaccination may indirectly favor the spread of yet rare but highly pathogenic serotypes.