The cause of viruses' appearance.
The whole historical picture of the development of modern 'virusology' reminds an endless race of survival. No matter how weird it sounds we should say that in this race, for now, viruses are those who survivals. Among thousands of viruses there are just a couple dozens of viruses that are unique and were created on the base of brand new ideas. More power to their creators, but what about us and those who fights against viruses. Because of the intensive development of virus' technologies antivirus programmers have to be extremely attentive and watch after the smallest changes at viruses' nature in order to secure our computer from the viruses' penetration. It's a pity but the efforts of antivirus programmers sometimes fail.
Let's recollect in our memory the events of 1991. The epidemic of notorious Dir-II has widely spread all over the network and damaged a great number of computers and a great volume of information and software. The virus' success was supplied by usage of brand new technologies for its creation. We shouldn't forget about Christopher Pain who have created the Queen and Pathogen. Both viruses are just a head of iceberg; Christopher created third, the most dangerous one – Smeg. The Smeg was able to collide over first two, which caused changed of Queen's and Pathogen's configurations after each run. Other words the Smeg made those two viruses impossible to detect and dispose. The mean of spreading these viruses were computer games and software that were infected with viruses and allocated in network. Internet users downloaded infected software and so infected their hard disks.
More over Pain infected the antivirus program, which was aimed to detect and dispose those viruses; thereby users who were trying to heal their computers with the help of antivirus instead of help got another virus. As a result milliards megabytes of information of different firms and organisations were erased and loss reached millions of pounds. Another 'evil genius' was programmer Morris, who created harmful virus and infected for about seven millions of personal computers that were connected to interned.
The reasons of creation of viruses a mostly psychological and depends on human's nature. Say, one wants to prove he is the best of the best, or to revenge someone who offended him, or just to play game for life with the best programmers of the world. The only way for him to draw their attention is to create something that will bother them and cause a lot of troubles, so he creates a virus to infect their computers. It is precisely this fact that explains why viruses are the survivors in this race for life, because virus creators constantly finds out new technologies to bit those the programmers have, but the programmers are to busy with creation of necessary software to watch after virus technology development, so they deal with virus problems as-it-comes.