The encapsulated bacteria Streptococcus pneumoniae (the pneumococcus), Neisseria meningitidis (the meningococcus) and Haemophilus influenzae type b (Hib) are the main causes of purulent meningitis, the peak incidence of which is seen in the first two years of life. The polysaccharide capsule of these bacteria is an essential virulence determinant, and antibodies to it are protective, suggesting that a polysaccharide vaccine could prevent these diseases. The young child is, however, unable to respond with antibody production to these polysaccharides, making such vaccines useless in infancy. Conjugation of the polysaccharide to a protein carrier has proven a way to solve the problem. Immunization of infants with such a Hib conjugate vaccine was shown in 1987 to result in the desired antibody production and protection from Hib meningitis and bacteremia. The Hib vaccine is now a part of national infant immunization programs in large parts of Europe, the Americas and Australia, and has resulted in the virtual disappearance of Hib disease from these areas. A group C meningococcal and 7-valent pneumococcal vaccine, available since 2000, are likewise proving highly effective in preventing bacteremic disease. Further advantages of the conjugate vaccines are their ability to elicit immunologic memory and to reduce asymptomatic carriage of the bacteria, resulting in marked herd immunity. This paper was delivered as a lecture in January 2003 in Bangkok on the occasion of the Prince Mahidol Award for a life’s work in the field of vaccinology. cause of other disease entities like otitis media, sinusitis and pneumonia. Recently also Hib has been shown to be an important cause of pneumonia in young children, accounting for an estimated 20% of their pneumonias with X-ray proven consolidation (Mulholland et al, 1997). A further common feature is that these bacteria often live on the mucosa of the nose and pharynx as parts of their normal microflora. This colonization or carriage is an important part of the chain of infection, ie transmission of the bacteria to new hosts.