Symantec doesnt detect virus

Hi,

We have an issue with regards to virus detection. One of the employees received a virus attachment from email. However, Symantec didnt detect this and some viruses can be detected by symantec when we open such attachment.

Is there any way that symantec can detect the virus even if the user doesnt open the attachment or a file downloaded from the internet? Or symantec can only detect viruses if the user open such files? I have doubt in symantec because it cannot detect all the viruses so if the behavior of this AV can only detect opened files, there's a possibility that a virus will infect our network if a user open a virus and Symantec cant detect this.