But while there may be no viruses as such in the wild (so we are led to believe) there most certainly are a massive number of exploits. Somewhere recently I was reading an article about compromised mail servers dishing out SPAM and a large number of *nix servers were detected.
Having been involved in tracking a couple script kiddies down and getting to watch their interviews it comes across that they target the MS operating systems because more often than not they want to teach a lesson to MS for one reason, they have a deep seated distain for authority or they want to show how talented they are. The last one is kind of funnyn as most of them are using tools created by someone else. They do however tend to run Linux distros and a couple have stated they do so because they can not infect their own machines.
The clowns that write virus or malware have varying reasons for why they do this with the single biggest one being they usually don't fit in with society and having spoke to a few people in the mental health area it comes across that it is often a deep seated need for recognition - even if that recognition is from other no-hopers.