The Linux servers occupy the largest share of this market, which means that they are the most prone to attacks from hackers and other malevolent cyber-criminals. ESET researchers and a few other agencies have shown that Unix servers have been used to spread malware and send spam emails.
As the teams from ESET, CERT-Bund, the Swedish National Infrastructure for Computing, and a few other groups have discovered, this concerted action from unknown cyber-criminal groups has been going on for quite some time.