More than 3 percent of District residents have been diagnosed with HIV/AIDS, a “modern epidemic” that D.C. officials say is likely much worse as upward of half of those infected don’t know they have the disease.

The number of known HIV cases among D.C. residents 12 years or older rose 9 percent between 2007 and 2008, from 15,120 to 16,513, according to an epidemiology report released Wednesday. Overall, 3.2 percent of the total population has been diagnosed with HIV, and that number will continue to rise as more people are tested, enter medical treatment and receive the care they need to stay alive.