Computer Viruses are programs which can copy it and infect the computer and other programs. Virus authors and antivirus software try to defeat each other. Here are some of the ways viruses use to defeat antivirus software
Stealth – Viruses can intercept the request by antivirus program to read files and then return uninfected clean file thereby fooling an antivirus program.
Self-modification – Antivirus software use know virus definitions to look for virus signatures (digitally identifiable virus information). However, some viruses can modify itself each time they copy therefore have a different variant, which can not be easily identified using virus signatures.
Encryption – Viruses can have varying encrypted module with a small decrypting module. That would make it difficult to identify viruses using known virus signatures.
Polymorphic technique – Polymorphic virus is similar to Encrypted virus. The difference is that in this case decrypted module also changes with every copy therefore make it even more difficult to identify virus using signatures.
Metamorphic technique – These viruses can write themselves completely making it extremely difficult to study the pattern.
Avoiding Bait files – Antivirus software sometimes creates bait files to invite viruses to infect these files. As viruses learn about such files they avoid infecting such files and other files that might be easily caught by anti-viruses.
Good antivirus software continue to research new threats and ways to combat them but there are times when virus authors can use a new and advanced technique before anti virus technology catches up. It is always advisable to use caution.