Viruses are in a constant battle with the host immune system. Viruses modulate both the innate and acquired immune systems using a number of clever strategies. The goal appears to be to survive within the host for a long time, rather than efficient replication. Excessive replication would lead to detection and elimination by the host innate and acquired immune systems, thus bringing about the death of the virus. The virus needs to strike a balance between activation and suppression of host immune response. It is likely that each virus has developed various strategies to modulate the host immune response individually, and viruses that have succeeded in creating a good balance between host and parasite have survived.