Exam 312-49v10 topic 1 question 191 discussion

You are running known exploits against your network to test for possible vulnerabilities. To test the strength of your virus software, you load a test network to mimic your production network. Your software successfully blocks some simple macro and encrypted viruses. You decide to really test the software by using virus code where the code rewrites itself entirely and the signatures change from child to child, but the functionality stays the same. What type of virus is this that you are testing?