Streptococcus pneumoniae (S. pneumoniae) bacteria, or pneumococcus, can cause many types of illnesses. These range from ear and sinus infections to life-threatening conditions such as pneumonia, bloodstream infections, and meningitis. Pneumococci are surrounded by a polysaccharide capsule, which is thought to help it evade the immune system. Presently, over 90 known serotypes of S. pneumoniae have been identified, of which only a minority produce the majority of pneumococcal infections; a serotype is defined by a unique pneumococcal capsule structure.