The 20th century was a period when computer viruses saw light and managed to cause serious damages to unprepared users and their machines. Millions of users downloaded computer viruses without even knowing about it. To download a computer virus one simply needed to click on a certain link or open an email attachment. Hackers used various ways of penetrating the systems of universities from around the world, NASA, armed forces and governments. In this article you will find the most dangerous computer viruses of the 20th century.