Why does, so called, HIV refuse to move outside its core risk groups?

<http://cityroom.blogs.nytimes.com/2008/01/02/why-hiv-cases-are-rising-among-youn
g-gay-men/>:

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Sarah Kershaw, who covers public health in New York City, examines the
reasons by this phenomenon: The number of new H.I.V. infections in men
under 30 who have sex with men has increased sharply in the city in
the last five years, particularly among blacks and Hispanics, even as
AIDS deaths and overall H.I.V. infection rates in the city have
steadily declined.

Among the key factors: higher rates of drug use among younger men,
which can fuel dangerous sex practices, optimism among them that AIDS
can be readily treated, and a growing stigma about H.I.V. among gays
that keeps some men from revealing that they are infected. (See a
related graphic and a blog post from September, when the Health
Department reported the increase in new H.I.V. infections.)

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