I have the same problem occurring across all of my sites. I am using Weaver ll Pro Theme. It seems that the only Plugin that is in common with those listed previously may be Akismet. Akismet was on a new install that I did for a customer on 31/8/2015. I removed Akismet straight away as I have found WP SpamShield does a better job, but it may have let the virus through.
