Host-based antivirus protection is also known as agent-based. Agent-based antivirus runs on every protected machine. Agentless antivirus protection performs scans on hosts from a centralized system. Agentless systems have become popular for virtualized environments in which multiple OS instances are running on a host simultaneously. Agent-based antivirus running in each virtualized system can be a serious drain on system resources. Agentless antivirus for virtual hosts involves the use of a special security virtual appliance that performs optimized scanning tasks on the virtual hosts. An example of this is VMware's vShield.