John works as a professional Ethical Hacker. He has been assigned a project to test the security of www.we-are-secure.com. He wants to test the effect of a virus on the We-are-secure server. He injects the virus on the server and, as a result, the server becomes infected with the virus even though an established antivirus program is installed on the server. Which of the following do you think are the reasons why the antivirus installed on the server did not detect the virus injected by John?

Each correct answer represents a complete solution. Choose all that apply.