CompTIA A+ Certification All-in-One Exam Guide

NOTE The term antivirus (and antispyware, or anti-anything) is becoming
obsolete. Viruses are only a small component of the many types of malware.
Many people continue to use the term as a synonym for anti-malware.
Antivirus programs use different techniques to combat different types of
viruses. They detect boot sector viruses simply by comparing the drive’s boot
sector to a standard boot sector. This works because most boot sectors are
basically the same. Some antivirus programs make a backup copy of the boot
sector. If they detect a virus, the programs use that backup copy to replace the
infected boot sector. Executable viruses are a little more difficult to find
because they can be on any file in the drive. To detect executable viruses, the
antivirus program uses a library of signatures. A signature is the code pattern
of a known virus. The antivirus program compares an executable file to its
library of signatures. There have been instances where a perfectly clean
program coincidentally held a virus signature. Usually the antivirus
program’s creator provides a patch to prevent further alarms.
EXAM TIP Be prepared for a question on the CompTIA A+ 1002 exam
about rogue antivirus software. This can mean one of two things. First, the
software might be a Trojan Horse, as we discussed earlier—a security
program that looks like it’s doing good for you but actually infects your
computer. Or, second, it could be horribly invasive anti-malware software
that protects your computer at the cost of making it not run well. You’re more
likely to see the first definition on the exam.
Now that you understand the types of viruses and how antivirus programs
try to protect against them, let’s review a few terms that are often used to
describe virus traits.