According to the Russian security software company, Dr.Web, a Trojan has succeeded in infecting over 550,000 Macs, the majority of which are located in the United States. The Trojan, named “Flashback”, takes advantage of a specific vulnerability in Java that was quickly addressed by Apple.
Read more here – http://averysawaba.blogspot.co.uk/2012/04/over-half-million-macs-infected.html