Meningococcal disease is caused by Neisseria meningitidis, a bacterium with human carriers as the only reservoir. It is carried in the nose, where it can remain for long periods without producing symptoms.
Poliovirus is highly contagious and infected individuals shed virus in the faeces and from oral secretions, thus the mode of transmission is person-to-person, both via the faecal-oral and the oral-oral routes.
Despite good access to effective antibiotics, Streptococcus pneumoniae (pneumococci) is still a major cause of disease and death in both developing and developed countries. Pneumococci are the main cause of bacterial respiratory tract infections, such as pneumonia, middle ear infection, and sinusitis, in all age groups.