This Russian city has a population of 1.5 million (which means, I guess, that Russian cities must be pretty small on average, if it's the fourth largest) and has 1.8% of its population with HIV. It's to do with drugs and (heterosexual) sex:
While the majority of new infections are acquired through intravenous
drug use, heterosexual sex is rising as a source of transmission and
accounts for just over 40 percent of new cases.
HIV infections in Russia are concentrated in large manufacturing
cities in southern Siberia and along drug trafficking routes that begin
in Central Asia and extend to Europe. Russia's Sverdlovsk Region, of
which Yekaterinburg is the capital, is the region most heavily infected
with HIV in Russia, according to Savinova.