How do computer viruses spread and in what ways do they affect computers?
Muhammad A. Khan

Computer viruses spread and in many ways they affect computers. A computer virus is a program written to enter your computer system and "infect" it by installing or modifying files or establishing itself in memory. Some viruses are benign and won't harm your system, while others are destructive and can damage or destroy your data. Computer viruses usually spread in one of following ways: from removable media; from downloads off the Internet; from email attachments; Online ads ; Malware and virus and encryption: Social media: Mobile malware : and User stubbornness.

There many means by which a computer virus may spread. Computer viruses may also spread from removable media. For instance, a person has a removable flash drive that has been infected with a computer virus from a source computer. Program or file viruses are software programs that attach themselves to executable programs. Once the infected program is run, the virus is transferred to your system's memory and may replicate itself further. It may spread to all the files on a hard drive or to files of a specific type. ("PC Hell: What Are Computer Viruses." PC Hell: Computer Hints and Tips to Bring You Back from the Edge. N.p., n.d. Web. 23 Apr. 2015.)

When the person boots the removable flash drive in another personal computer it will trigger the spread of the virus from the removable flash drive to the personal computer. Any person therefore who inserts a floppy disk or a removable flash drive in the infected personal computer may also get a computer virus. According to pctools, Many viruses attach themselves to legitimate executable files on a host computer that allow the virus to be loaded when the user opens the file. (http://www.pctools.com/security-news/what-do-computer-viruses-do/) The transfer of virus will continue until the computer virus is detected. Or, when a person purchases software that is infected with a computer virus and then uses the same software in his personal computer the running of the software can trigger the virus to spread to this personal computer.

There are many common computer viruses that are spreading nowadays. One of the most common viruses is called the Win32/Virut.NBK. This virus is a malicious and destructive program that sneaks inside the systems such as the XP or Vista. According to Microsoft website, this virus is a polymorphic file infector that targets .exe and .scr file. It is capable of modifying the files names and installing various files on the infected machine. It can also create corrupt .exe and .dll files and download further malware threats onto the system. The common source of this virus are the adult web sites which allow its users to download films, infected email attachments or through file sharing. This virus is so destructive that Microsoft considers that the alert level for this virus is severe.

Another way is through email attachment. A Trojan horse is different from a computer virus or computer worm because it cannot replicate itself or reproduce. It can spread from one computer to another through a variety of means. It can be transferred through email attachments. Once the recipient who receives the computer attachment opens the attachment, the virus can immediately infect the system. It can also be spread by means of chat software such as Yahoo Messenger or Skype. Another method is by sending copies of itself to the people in the address book of the user who has a Trojan horse virus.One of the means to protect a user from a Trojan horse virus is by not opening email attachments or files from which have been sent by a unknown user. Another is by updating the user’s anti-virus software.

Malware and virus obfuscation and encryption is a New malware can bypass antivirus tools when they are modified with encrypted, or obfuscated. For example, let's say we have "malware.exe,"