Thats 50% of the (for lack of better terms) native population- not including those here temporarily for college, internships, congressional terms, ect... Those statistics apply more or less to the African-American population (most of whom have been in DC for at least a few generations), and also the homosexual population (who may or may not be native Washingtonians). Together, both of these groups make up a large majority of DC's population. And it is estimated that a scary percentage of them (I've read anywhere from 27-70%) don't even know that they are HIV+.
Geographically, DC is not a large city, and, (although housing prices are astronomical in most parts) not as physically crowded as some other cities (like Tokyo). But DC has had the highest per capita HIV rates- higher than even San Francisco. Thats because in S.F., people are very pro-active in knowing their "status", and therefor there is a lower rate of transmission.
I know this sounds bad, but at ANY hospital in DC city limits (including Vetrans Hospital, university, public, or private hospitals), when African-Americans (or certain immigrant groups), or any homosexual male, or any IV drug user, shows up in any of the hospitals, we know that probably 50% are HIV+, and many don't know it or may not admit it.
Yeah, DC is pretty ghetto. These hospitals are also the same ones that Congress and White house staff go to.