Viruses

A virus is malware that hides inside another file or program. It attaches its own code to that host file, then copies itself into other programs once it gets the chance to run.

Why a virus needs you

A virus cannot move about on its own. It sits inside its host file doing nothing until a user opens the document, launches the program or runs the installer it is attached to. Only then does the virus code start running. That is why viruses are often tucked inside files that look worth opening, such as a downloadable recipe collection, a shared club spreadsheet or an attachment pretending to be a ticket.

Once the infected file is opened or launched, the virus searches for other files and programs it is able to write to, and writes a copy of itself into them. Anyone who later opens one of those newly infected files starts the process again, so one careless open can spread the virus through a whole shared folder.

What a virus does to a computer

An active virus harms the computer by interfering with the files stored on it. It may delete files so they are removed from the system, corrupt them so they will not open or the data inside is damaged, or modify them so they still open but the contents are wrong. Because the virus changes files rather than breaking the hardware, the machine usually still switches on and behaves normally at first, which makes the damage easy to miss.

Imagine a virus hidden in a recipe file you download to plan a family meal. When you open it, the virus copies itself into the other recipe files in the same folder and changes the oven temperatures in two of them, so the dishes no longer cook correctly. Nothing looks wrong at the time; the errors only appear days later when someone follows the recipes.