ZyXEL Communications USG40 User Manual page 679

Computer Virus Infection and Prevention
The following describes a simple life cycle of a computer virus.
A computer gets a copy of a virus from a source such as the Internet, e-mail, file sharing or any
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removable storage media. The virus is harmless until the execution of an infected program.
The virus spreads to other files and programs on the computer.
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The infected files are unintentionally sent to another computer thus starting the spread of the virus.
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Once the virus is spread through the network, the number of infected networked computers can
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grow exponentially.
Types of Anti-Virus Scanner
The section describes two types of anti-virus scanner: host-based and network-based.
A host-based anti-virus (HAV) scanner is often software installed on computers and/or servers in
the network. It inspects files for virus patterns as they are moved in and out of the hard drive.
However, host-based anti-virus scanners cannot eliminate all viruses for a number of reasons:
• HAV scanners are slow in stopping virus threats through real-time traffic (such as from the
Internet).
• HAV scanners may reduce computing performance as they also share the resources (such as CPU
time) on the computer for file inspection.
• You have to update the virus signatures and/or perform virus scans on all computers in the
network regularly.
A network-based anti-virus (NAV) scanner is often deployed as a dedicated security device (such as
your ZyWALL/USG) on the network edge. NAV scanners inspect real-time data traffic (such as E-
mail messages or web) that tends to bypass HAV scanners. The following lists some of the benefits
of NAV scanners.
• NAV scanners stops virus threats at the network edge before they enter or exit a network.
• NAV scanners reduce computing loading on computers as the read-time data traffic inspection is
done on a dedicated security device.
Chapter 38 Anti-Virus
ZyWALL/USG Series User's Guide
679