Malware

Malware is short for malicious software: programs written to cause harm to a computer system, damage or steal data, or gain unauthorised access to it. It is software rather than a physical fault, so it must arrive as code that has got onto your computer in some way.

Malware only works once it runs

Malware is executable code, which means the processor has to carry out its instructions before anything happens. A file sitting in a downloads folder or on a memory stick is only stored data; on its own it cannot change, delete or send anything.

This is why getting the code to run is the whole aim of an attacker. Once it runs, the malware works with the permissions of whoever launched it: code started by an ordinary user account can read and delete that user's documents, while code started by an administrator account can reach far more of the system. Malware that manages to run can also keep running quietly in the background, so a person may not notice anything until files or data go missing.

For example, Priya saves a file from a message and leaves it unopened in her downloads folder for a week, and nothing at all happens. The moment she double-clicks it, its instructions execute and it begins deleting the photos in her pictures folder. The same file, never opened, would have caused no damage.