SUBMICROSCOPIC INFECTIOUS agents have a way of revealing the worst in us, and the best. That is the story of the AIDS epidemic generally, and in particular of ACT UP — the 33-year-old radical direct-action group formally and loftily called the AIDS Coalition to Unleash Power. For nearly a decade in the 1980s and 1990s, ACT UP was a ubiquitous and unnerving presence, not only in America but in 19 countries worldwide. At its peak, it claimed 148 chapters, and though its ranks remained relatively small — numbering perhaps no more than 10,000 — it terrified and angered much of the population, whether by halting rush-hour traffic and taking over public spaces with “die-ins” and “kiss-ins,” at which members laid on the ground or made out with one another or by disrupting scientific conferences and political affairs with foghorns, fake blood and smoke bombs (even, in one instance, overturning banquet tables). Generally, the news media didn’t think much of their work, branding the group both vulgar and counterproductive. “Far from inspiring sympathy,” The New York Times said of ACT UP in 1989, their methods were “another reason to reject both the offensive protesters and their ideas.”