Who writes viruses and why do they do it?
Viruses are created by programmers, both amateur and professional. Many of them are young (under 20 years of age) and it is arguable that they are not always aware of the impact of their actions in releasing viruses 'into the wild'. Sometimes they create viruses simply to show off their programming skills to the world, the same way an unthinking teen will spray-paint a 'graffiti tag' on a derelict building. Unfortunately, the power of the Internet is such that the familiar teenage impulse towards minor vandalism has a massively magnified impact when it takes the form of a computer virus.
Significantly, however, viruses are increasingly being created and manipulated for illegal commercial gain by mature individuals well aware of the criminal implications of their actions. These individuals use viruses to steal information and money. And there are still other virus writers are simply malicious, seeking only to harm other people's property and livelihoods.
Why does Windows have so many viruses?
One of the most common forms of virus is the macro virus, which takes advantage of the 'higher level' programming tools (macros) that are part of Microsoft Office programs such as Microsoft Word and Excel. Virus writers have easily been able to insert viruses into these 'higher level' functions, which accounts in part for the vast number of viruses that target Microsoft software. Additionally, Microsoft's tendency to bundle its software programs (both commercially and technologically) makes it easier to exploit known loopholes in one program and take control of others in the same programming family. Finally, Microsoft's market penetration have made the company a natural target.
In contrast to the many viruses that plague the Microsoft Windows platform, Linux and Macintosh operating systems have proven far more difficult to attack, although they are not completely immune to viruses either.