WannaCry

Well Kaspersky Lab has recently showed that almost all documented WannaCry infections are running Windows 7 operating systems.  

Although we have not yet confirmed it, a computer is being examined by our engineering department and has been found with a most incredible virus. Probably the most incredible one we have seen. Its antivirus  software Avast was up to date and stated that it was protecting the computer. The Avast software was able to do a quick search for virus, which showed nothing. It could not however run a full scan. What alerted the operator that something was wrong was when he followed my advice and tried to run Bitdefender Anti-Ransomware and it did not allow this software to run. Then when he tried to run a competitors antivirus software, the computer crashed. We have never before seen such a sophisticated virus. Whether its WannaCry we do not know as yet but its a strong possibility.

More details here.