Viruses often produce proteins that trigger apoptosis of host cells. This promotes the spread of infection and can subvert the host's immune response. The HIV envelope protein (Env) is thought to promote apoptosis of infected cells by several mechanisms. Guido Kroemer and co-workers now describe how it can also cause death of uninfected `bystander' cells following cell fusion (see p. 5643). Fusion-induced death in which cell cycle stalling slowly leads to apoptosis has been seen before. What is new is the rapid `contagious' apoptosis the authors observe when uninfected cells encounter preapoptotic Env-expressing cells. They show that this exhibits typical apoptotic features (e.g. loss of mitochondrial potential and caspase activation) but is unaffected by various inhibitors that block classical fusion-induced death. Significantly, Kroemer and co-workers show that contagious apoptosis occurs both in co-cultured cell lines and in vivo when they inject Env-expressing cells into nude mice. Their findings raise the possibility that the new phenomenon is responsible for `bystander killing' of healthy CD4+ T cells in HIV-infected individuals and thus contributes to the development of AIDS.