Much of the information previously released has focused on the Joel/Ellie relationship and gameplay relating to gangs of human survivors of the plague.
However, we've now got a bit more info on those who have been infected. Whilst The Last Of Us is a zombie apocalypse game, its zombies are not the standard fare. Instead, they've been infected by a fictional version of a real fungus (which afflicts little creatures such as ants). Early stage 'zombies' run around (and are thus known as runners) trying to infect everyone they can. Happily, they're not too hard to kill.
Once the fungus progresses, though, things get more dangerous, and rather creepier. The fungus consumes the eyes and upper head of its victim, rendering him blind. He therefore finds the player not by sight but echo-location (hence the designation of clicker). They're much harder to kill (and remember, kids, in The Last Of Us bullets are uncommon) and can also kill the player very quickly. And they sometimes hunt in packs.
From gameplay videos I've watched fighting the zombies does look very tense and atmospheric. However, it's worth pointing out that the clickers look pretty repugnant, and far more unpleasant than the standard zombie.
There's been relatively little fungus/zombie information put out, and recent information does emphasise the survival horror dimension of the game. So if you're squeamish, you might want to give this one a miss.