New research has allowed San Francisco’s health department to map out where HIV/AIDS care should be targeted, The New York Times reports. The map uses data from individuals’ viral loads to identify these treatment gaps and show where the virus is circulating.

The map illustrates that some areas like the Castro—the city’s gay epicenter—have more HIV cases, but that individuals in low-income neighborhoods such as Potrero Hill and Bayview have the highest viral loads and are in more need of treatment.

Grant Colfax, MD, director of HIV prevention and research in the city’s health department, calls the map, which tracked viral loads from 2005 through 2007, “a thermometer” for measuring the city’s HIV epidemic. “We’re taking an individual marker and making it a marker for community health,” he said.

While other communities have mapped out their respective HIV epidemics, those efforts have relied on counting HIV/AIDS cases and do not take viral load into account.

Julio Montaner, MD, president of the International AIDS Society and head of the division of AIDS at the University of British Columbia, said viral load mapping is important because “you can identify hot spots where, in all likelihood, most transmission is occurring.”

He added, “These hot spots are perpetuating themselves, increasing infection in marginalized communities. This is unacceptable. As long as we don’t deal with that problem, the reservoir of HIV will ensure that we’re promoting the continued spread of HIV in perpetuity.”