英语翻译Antivirus software uses several techniques to find virus

英语翻译
Antivirus software uses several techniques to find viruses .Some viruses (attach) themselves to an exising program .The presence of such a virus often increases the length of the (original) program .The earliest antivirus software simply examined the programs on a computer an recorded their (length) .A change in the length of a program from one computing session to the (next) indicated the possible presence of a virus.