Explain why Peter had an amnesia ? and Sylar become very weak ?

If You Haven't Watched Season Two You Will Find Out By The End & The Following Will Be Spoilers.

The Haitian Took Peters Memory & Sylar Has The Shanti Virus Which Takes Away Powers After A While & He Was Stabbed In The Chest With A Samurai Sword.

the haitian wipe peter's memory & sylar became weak because he died in season 1 means his wounds won't let able to use his abilities