In the study among Navy recruits in the United States, almost 300,000 recruits had resided all their life in a single County (the US has over 3,000 counties). These recruits were given a sensitin produced in an identical manner as tuberculin, but made from M. intracellulare (which belongs to the M. avium complex). It was denoted PPD-B (The "B" is from Battey Hospital in Rome, Georgia, where the organism came from).

The darker the area, the higher the percentage of reactors to PPD-B. In the south and along the southern and central parts of the east coast, a large proportion of recruits reacted to PPD-B while such reactions were rare or absent in the north-west and along the Canadian border.

This explains why recruits from Minnesota had much less frequently small reactions than recruits from North Carolina. The small reactions found in North Carolina are interpreted as non-specific cross reactions resulting from infection with environmental mycobacteria.

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