The scale of the collective mobilization sparked by the AIDS epidemic has been highlighted time and again. Due to a logic of selective dissemination, infection with the human immunodeficiency virus (HIV) has not only spurred action among patients and their loved ones—as is usually the case with diseases—but also among the two “social groups” most affected: gay men and people who use drugs. Existing for nearly a century, “patient” collectives encompass diverse configurations, ranging from consensual groups that developed in the 1930s to more protest-oriented groups that emerged during the 1970s. Here we will show how, in the fields of AIDS and drug addiction, these user collectives have proliferated through choices of differentiated public self-identification. In the field of the fight against AIDS, all forms of mobilization previously found in the realm of disease coexist. In the field of drug use, the approaches to action are less diverse and fall into two broad categories: interest groups and self-help groups.