When you use a web based email service like gmail, you rely on the service provider to protect you from virus infection. Unlike spam protection, you never need to be consulted by any filter ("would you like to be infected by this virus that I found in your email?" - Yes my name is George or No), you just ditch the message.
This article - Webmail Email Virus Protection Comparison, shows that Google is doing the right thing by protecting users completely where it can. And even though Hotmail manages to do the same by enforcing "thou shalt not have attachments" rules, infected files still get through.
Interestingly it appears that Norton beats McAfee at finding virus infected files.