3d illustration of Streptococcus pneumoniae bacteria (pneumococci). These Gram-positive spherical bacteria colonize the respiratory tract asymptomatically in healthy carriers but can cause pneumonia and are one of the causative agents of bacterial meningitis in individuals with weaker immune system, particularly in the elderly and young children. They can also cause a range of infections in different locations, such as otitis media, conjunctivitis, endocarditis, sepsis and osteomyelitis.