How did Bob get the jump on Brad and bite him?

That part seemed weird, i mean Bob was at a distance and Brad had his gun aimed right at him prepared for when he turned, yet he gets bit? I mean he survived a yard full of viral's at the compound, jumping and leaping from everywhere, yet this big fat old dude gets him while he's got his gun aimed right at him at a distance?

Kinda seemed like a cheap way to get Brad infected so he can be in the show when its set in the future. I have no problem with that, but i feel like they could have gotten him infected in a less dumb way. Like one of them just gets him from behind when his guards down, not when he's clearly staring at the infected at a great distance fully prepared to shoot it. Just seemed dumb after all he's survived thats how he gets bit.