post spam

I have been getting a lot of post spam. By “post spam,” mean that someone/something is regularly and automatically inserted chunks of HTML into the body of existing posts. The post itself is never altered or deleted and this new code is always inserted below in an element (usually a div) with an inline style set to display none. I have searched the net and the WordPress forums and have been quite surprised that I have not found anything on the matter. I find it hard to believe that I am the only one experiencing this. Because of that I have wondered if my server has been hacked and the exploit is coming from inside. I have worked tirelessly to ensure that that is not the case however and have yet to find anything substantial. So, I am desperately reaching out for an answer. Has anyone else experienced post spam? Has anyone heard of this? Does anyone know how they are getting in?