The choice is really between "bridge groups" and "transitional groups" with regard to HIV transmission.

It appears "bridge" is more commonly used...

USAID Guinea / Health/ News: Guinea HIV Country Strategy Approved
The Guinea Mission’s HIV/AIDS Country Strategy was officially approved by ...
the population that may serve as bridge groups to the rest of the population. ...
www.usaid.gov/gn/health/news/030623_hivstrategy/ - 18k - 16 Nov 2005 - Cached - Similar pages

AIDS IN ASIA / China Becomes Proactive / Beijing wakes to health ...
Beijing now acknowledges 1 million cases of HIV/AIDS, up from 600000 in 2001.
... the general population" because of these bridge groups, said Siri Tellier, ...
www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/...

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Note added at 7 hrs 18 mins (2005-11-17 15:43:59 GMT)
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The concept of "bridge groups " which connect individuals with "high-risk" behaviors to the general population and the need to target prevention interventions to these groups was also discussed. Professor Mboup identified two groups in Senegal as examples of these "bridge groups": migrant workers and fishermen. Professor Kanki emphasized the need to identify similar kinds of "bridge groups" in Nigeria in order to adequately target prevention interventions.
http://www.hsph.harvard.edu/apin/report-testingsurveillance....