In order to avoid detection by users, viruses programs use different kinds of deception techniques. Older viruses, especially viruses designed for MS-DOS made sure that the “last modified” date of a host file stayed the same when the file was infected by the virus. This approach does not fool anti-virus software, however, especially those which maintain and date Cyclic redundancy checks on file changes.

Some viruses can infect files without increasing their sizes or damaging the files. They accomplish this by overwriting unused areas of executable files. These are called cavity viruses. For example, the CIH virus, or Chernobyl Virus, infects Portable Executable files. Because those files have many empty gaps, the virus, which was 1 KB in length, did not add to the size of the file.

Some viruses try to avoid detection by killing the tasks associated with antivirus software before it can detect them.

As computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced. Defending a computer against viruses may demand that a file system migrate towards detailed and explicit permission for every kind of file access.

Computer viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation.

The term “virus” is also commonly but erroneously used to refer to other types of malware, adware, and spyware programs that do not have the reproductive ability.

A virus might corrupt or delete data on your computer, use your e-mail program to spread itself to other computers, or even erase everything on your hard disk.

Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.

Computer viruses are often spread by attachments in e-mail messages or instant messaging messages. That is why it is essential that you never open e-mail attachments unless you know who it’s from and you are expecting it.

Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files.

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