Tens of thousands of viruses and malware exist, with more being created each day. Although once most common in Microsoft™ DOS™ or Microsoft™ Windows™, computer viruses today can cause a great amount of damage by exploiting vulnerabilities in corporate networks, email systems and Web sites.
Virus and malware types:
Joke program: A virus-like program that often manipulates the appearance of things on a computer monitor
Trojan horse: An executable program that does not replicate but instead resides on systems to perform malicious acts, such as opening ports for hackers to enter. A Trojan program often uses ports to gain access to computers. An application that claims to rid your computer of viruses when it actually introduces viruses onto your computer is an example of a Trojan program. Traditional antivirus solutions can detect and remove viruses but not Trojans, especially those already running on the system.
Virus: A program that replicates. To do so, the virus needs to attach itself to other program files and execute whenever the host program executes.
ActiveX™ malicious code: Code that resides on Web pages that execute ActiveX™ controls
NOTE: does not apply to Macintosh™ computers.
Boot sector virus: A virus that infects the boot sector of a partition or a disk
COM and EXE file infector: An executable program with .com or .exe extension
NOTE: does not apply to Macintosh™ computers.
Java malicious code: Operating system-independent virus code written or embedded in Java™
Macro virus: A virus encoded as an application macro and often included in a document
VBScript, JavaScript or HTML virus: A virus that resides on Web pages and downloaded through a browser
Worm: A self-contained program or set of programs able to spread functional copies of itself or its segments to other computer systems, often through email
Test virus: An inert file that acts like a real virus and is detectable by virus-scanning software. Use test viruses, such as the EICAR test script, to verify that your antivirus installation scans properly.
Packer: A compressed and/or encrypted Windows™ or Linux™ executable program, often a Trojan horse program. Compressing executables makes packer more difficult for antivirus products to detect.
Network viruses
A virus spreading over a network is not, strictly speaking, a network virus. Only some of the virus/malware mentioned above, such as worms, qualify as network viruses. Specifically, network viruses use network protocols, such as TCP, FTP, UDP, HTTP, and email protocols to replicate. They often do not alter system files or modify the boot sectors of hard disks. Instead, network viruses infect the memory of client computers, forcing them to flood the network with traffic, which can cause slowdowns and even complete network failure. Because network viruses remain in memory, they are often undetectable by conventional file I/O based scanning methods.