For brand new virus, the best antivirus also will missed it(even it is able to detect some unknown/future viruses).

We refer to "clean" instead of "kill". Kill a virus is easy, just find out and delete the infected file. However, cleaning a virus is much more complex task. It is about restore the infected file back to its original stage.

Therefore, many infected files are not cleanable and that's why Antivirus software quarantines the infected file and let you to decide whether kill(delete) them or not.

Quarantined virus is being jailed by Antivirus. No able to spread. That means you are not able to access the file, too.

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LiewCF | Malaysia Bloggers Forum