Across all ethnicities, in the U.S. the group most affected by HIV infection is gay and bisexual men and other men who have anal sex with men. Among men who have sex with men (MSM) Black gay men have much higher rates due to smaller sexual networks, which has led the Centers for Disease Control and Prevention to warn that, if things don't change, one out of every two Black MSM will be diagnosed with HIV in their lifetime. The lifetime risks for Latino MSM is one in four versus one in 11 for white gay and bi men.
Transgender women, especially women of color, are at a significantly higher risk as well, though exact numbers aren’t known because many studies erroneously include them in the MSM category. However, a 2014 World Health Organization report (a meta-analysis of data from 15 different countries) found transgender women were nearly 49 times more likely to have HIV than the general population.
Injection drug users have higher risk rates as well. Different behaviors and sexual acts have different risks too. The CDC has a great breakdown comparing those risks here. For example, in anal sex, the receptive partner has greater risk of becoming HIV-positive than the insertive partner; and anal sex has higher risks than vaginal sex. Those who don't use condoms are more at risk than those who do — but, and its a big but, that's not true if both partners are on PrEP, the HIV prevention treatment which prevents 99 percent of HIV transmissions. Likewise if one partner is HIV-positive but has an undetectable viral load, they have zero chance of transmitting HIV to their partner. Yes, zero.