What is it about a Trezor that means it can't be infected by malware? The Trezor testimonials include that 


  TREZOR ... cannot be infected by malware 


What is it about a Trezor that means it cannot be infected?
Trezor is a relatively simple, specialized hardware device. It does not run any OS, just a small program specifically developed for signing Bitcoin transactions and managing the keys. Thus, the attack surface is very small compared to conventional general-purpose computers. You can say it's practically impossible to infect it.

Security is a much more complicated area though and I wouldn't say it's impossible unless I can formally prove it. It is however the safest device I know of (but this is subjective).