As the world becomes more electronically connected, systems
running on networked computers become more vulnerable to digital
threats and this has posted a serious challenge for information
security. On the Internet, anyone can send any kind of packet to
anyone without being authenticated, while the receiver has to process
any packet that arrives to a provided service. The lack of
authentication means that attackers can create a fake identity and
send malicious codes with impunity. All systems connected to the
internet are potential targets for attacks since the openness of the
internet makes them accessible to attack malicious objects. The
propagation of such objects from main server to any of the computer
nodes may be via vertical transmission in the network. In
this condition, the anti-virus software is run only to new infected
nodes. It has been investigated that there is no permanent recovery
from the malicious codes till a node is attached to the network,
even after the run of anti-virus software.