Real-time protection, on-access scanning, backgroun guard, resident shield, autoprotect, and other synonyms refer to the automatic protection provided by most antivirus, antispyware, and other antimalware programs, which is arguably their most important feature. This monitors computer systems for suspicious activity such as computer viruses , spyware, adware, and other malicious objects in 'real-time', in other words while data is coming into the computer (for example when inserting a CD, opening an email, or surfing the web) or when a file already on the computer is opened or executed, in other words loaded into the computer's active memory. This means all data in files already on the computer is analysed each time that the user attempts to access the files. This can prevent infection by not yet activated malware that entered the computer unrecognised before the antivirus received an update. Real-time protection and its synonyms are used in contrast to the expression "on-demand scan" or similar expressions that mean a user-activated scan of part or all of a computer.