Which pair of activities accounts for around half of all new HIV cases in the United States?

The selected answer highlights the significant impact that men who have sex with men (MSM) and injection drug usage have on the transmission of HIV in the United States. This demographic is particularly affected due to various factors, including higher prevalence rates of the virus within these groups, increased likelihood of risky sexual practices, and the role of sharing needles among injection drug users.

Research indicates that MSM accounts for a large percentage of new HIV diagnoses, largely due to network effects, where the virus circulates more rapidly within connected communities. Meanwhile, injection drug use remains a pivotal risk factor due to the direct transmission of the virus through shared needles, which can facilitate rapid spread.

While other activities listed have contributed to HIV transmission, they do not collectively account for as significant a portion of new cases as the combination of MSM activities and injection drug use. Thus, this pair encapsulates key risk behaviors directly linked to the disproportionate rates of new infections observed in the United States.