General use of vaccines against Streptococcus pneumoniae (S. pneumoniae) in infants has led to a decrease in the presence of the serotypes against which the vaccine was developed. It is a pathogen of which more than 90 different serotypes exist, of whom 10 to 13 are covered by the current vaccines. In order to gain a clear understanding of the S. pneumoniae serotypes carried by infants, and the impact of the vaccine used in the vaccination program, the current study was set up. As early as the first months of our lives S. pneumoniae is present in the nasal cavity and in the pharynx, mostly only temporary and innocuous, yet in some cases, in infants it may lead to infections such as otitis, pneumonia or meningitis. Since the presence of S. pneumoniae is more abundant in some circumstances, the current study focuses on infants between the age of 6 and 30 months either healthy and residing in day-care centres or suffering from an acute middle ear infection. Over a 4 year period, a swab will be taken from the nasal cavity of these infants (700 in first year, 900 in subsequent years, 6800 in total) to investigate presence, density and type of S. pneumoniae carriage, if any, as well as its resistance against antibiotics. The presence of some other pathogens that can cause airway or ear infections will be investigated too. The findings of this study will be extremely valuable to guide decisions on vaccine development, vaccine program, and recommendations on antibiotic treatment.