Immunology
A natural immunity or resistance to cholera seems to exist because not all persons ingesting infected material contract the infection. C. Macnamara (1870) noted that of 19 persons accidentally drinking infected water on shipboard in 1861, only 5 contracted the infection.
This resistance might well be nonspecific; the natural acidity of the stomach could act as a barrier to infection (a possibility well appreciated by Macnamara). On the other hand, one infection does not produce the solid immunity against repeated infections in survivors as in other infections such as smallpox and measles. When postinfection immunity does arise, it is of relatively short duration. Extensive experimental and clinical trials with immunization against cholera have not been convincing. Such measures have been mandated at times for travelers, nevertheless. The weakness of postinfection immunity is consistent with the weakness of the case for immunization.
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