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. complete story here http://news.softpedia.com/news/quot-Operation-Windigo-quot-Attack-Infects-10-000-Unix-Servers-Millions-of-PCs-at-Risk-432920.shtml
more details are here http://blog.eset.ie/2014/03/18/operation-windigo-malware-used-to-attack-over-500000-computers-daily-after-25000-unix-servers-hijacked-by-backdoor-trojan/
and here http://thehackernews.com/2014/03/operation-windigo-linux-malware.html