Sirius Black and Lupin misunderstanding?

In Harry Potter and the Prisoner of Azkaban, I was watching a part where Lupin turns into a werewolf and attacks Potter. Now, I don't understand the fact that Sirius was attacked by Lupin, yet Sirius doesn't turn into a werewolf himself.

And I have another question: Why in the Hell didn't Harry, Ron or Hermione just apparate out of the forest when the were getting chased by the Snatchers in Harry Potter and the Deathly Hallows part 1?Sirius Black and Lupin misunderstanding?
Sirius doesn't turn into a werewolf because Lupin didn't bite him. He just... attacked him. People must get bitten by a werewolf to turn into werewolves themselves.
About the other thing... well, maybe the person above is right, they wouldn't be able to find each other since they couldn't say out loud where they were going to apparate, so they would apparate in different places...Sirius Black and Lupin misunderstanding?
Okay, I'm a loser but I might have your answer!! I think that Sirius got attacked exactly because he wasn't a werewolf, but in the book it says that when they change in to other animals they are safe from him. If I had to take a guess, it would be because Sirius went for Lupin aggressively so Lupin retaliated. That's my best guess for that one! And for theatrical drama. Everyone likes a good fight...
As for the second, in a state of panic the three of them probably couldn't gather themselves to apparrate safely. Also, they weren't able to grab on to each other so they could all end up in the same place. If they apparated separately then they would be lost from each other. And added on top of that, Rowling probably needed to add something interesting in there! Otherwise it would have gone on forever wouldn't it? That situation helped unravel some interesting things.
Oh and just to clarify, Sirius can ONLY turn into a dog and they all knew how to apparate by the last book :)Sirius Black and Lupin misunderstanding?
I don't think Lupin attacks Sirius in the books, so that's probably why he didn't turn into a werewolf (it would just be too off the path of the books if he did). Harry and the gang didn't know how to apparate until later in the series, after they'd been trained to do it.

Edit: Sirius could have turned into a werewolf if he was infected. No where in the books or movies does it say that because he's learned how to turn into a dog he can't become a werewolf. The whole reason he, and the rest of his childhood friends, learned how to become animagi was to prevent Sirius from attacking him. Hence why the attack of him doesn't make sense in the movie.
First off, Sirius can't turn into a werewolf. He is an animagus.

He can turn into any animals but he usually chooses that dog like creature.

Also, to answer your other question, I have no idea why they didn't do that. I guess they couldn't get so far away from the snatchers to apparate away from where they were without the snatchers actually apparating with them.