Big Companies = Big Mistakes
Studies are showing that big businesses are still failing to comply with security protocol that have been developed to prevent such problems. From portable hard drives to the most troublesome flash disks, malware is still getting into these big companies even with heavy investment in security software.
The most detrimental factor is the attitude of people who are the weakest links in the IT security equation. Complacency and utter disregard for security protocol such as scanning drives before you use them and scanning again when transferring from computer to computer is to blame. People bring their drives home to finish off work using an infected computer thereby transferring malware in an attack from within once they get into their office PC.