Apoptosis is an active process of cell death that serves diverse functions in multicellular organisms, and under physiological conditions, it is tightly controlled. Many virus genomes encode gene products that modulate apoptosis, either positively or negatively, and induction of apoptosis often contributes directly to the cytopathogenic effects of the viruses. Inhibition of apoptosis by viruses, on the other hand, may prevent premature death of infected cells, thereby facilitating viral replication, spread, or persistence.