From ND Forums, a user:
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The Runners: Stage 1 Infection
They are very fast and attack in packs. Neil Druckmann states ""They can still see, and they still have a sense of humanity. They almost know consciously what they're doing, but they can't control themselves. "They run fast, they can take you down, and they work in packs." They don't pose too much of a threat by themselves but it's a different story if confronted by many.
They can still see you so it's best to avoid their line of sight. Turning off your flashlight around them will help. These are more like the Infected from 28 Days Later.
??? : Stage 2 Infection
There isn't any info on the second stage of infection yet but I would assume the picture below might be it. As far as their abilities, my guess is that they are just as fast as the Runners and release spores into the air seeing how Joel has a gas mask.
The Clickers: Stage 3 Infection
These are the same types as the ones seen in the VGA 2011 debut trailer. "As the fungus spreads, it takes over more and more of the biology of the human. It grows out of the eye sockets, and it makes you go blind. This is a fate worse than death," Druckmann explained. "You're still alive, but you have to suffer through this pain."
"They're called clickers because they use echolocation. They scan the environment using sound waves… if there's any movement detected, they're on you," Druckmann described. "They might be a little slower, but when they're on you, they're going to be rabid. When they touch you, you're gone."
The Clickers are blind and cannot see you so it's best to distract them by throwing bottles and sneaking past them. You can use a Shiv to easily sneak up on them for an instant silent kill. Taking them out one by one will be easier but it's best to avoid them when necessary.
Eventually, the fungus takes over the body completely. In the gameplay demo that was recently played, "a body was grafted onto the wall, as the bulbs outstretched beyond the body" said a previewer.
From Kotaku,
"Before leaving Druckmann, I ask if there's anything beyond the Clickers, a level of infection even more terrifying than Stage 3. Like a man who takes a bit too much pleasure in scaring the pants off people, he shoots me a sly look and says "Yeah, there's more stuff...oh yeah."
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