The organisms which thus grow on the surfaces have in some cases been shown to be of diminished virulence, but in others have full pathogenic power.

Such passive carriers of infection have been found for a number of diseases, as cerebro-spinal meningitis, diphtheria, poliomyelitis and cholera.

In all these cases the organisms are most frequently found in those individuals who have been exposed to infection as members of a family in which there have been cases of disease.

The other sort of carrier has had and overcome the disease, but mutual relations have been established with the organism which continues to live in the body cavity.

Diphtheria bacilli usually linger in the throat after convalescence is established, and until they have disappeared the individual is more dangerous than one actually sick with the disease.