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.
Smallpox was a systemic disease, officially eradicated since 1979 (WHO), caused by infection with the Variola major virus, whose only reservoir was infected humans.
Genital chlamydia is the leading sexually transmitted infection in Europe and the cause of considerable acute morbidity and long term reproductive health problems, particularly in young people.