FAQ

In terms of chronology, Metroid Fusion is the last chapter in the Metroid series. In order:

Metroid (Zero Mission) - Metroid Prime - Metroid Prime: Hunters - Metroid Prime 2: Echoes - Metroid Prime 3: Corruption - Metroid II: The Return of Samus - Super Metroid - Metroid: Other M - Metroid Fusion

So Metroid Fusion takes place an unknown amount of time after Metroid: Other M.

Samus' suit actually consists of a mechanic part (the metal outer shell) and an biological part (an inner organic layer). The inner layer is the suit's interface and is in direct connection with her skin. Samus can move the suit because the inner layer 'reads' the commands given by her central nervous system, and relays the commands to the mechanic part.

The infection with the X parasite also spreaded to the organic parts of the suit, causing the inner layer to be fused with Samus' body. The only way to remove the suit was to surgically remove parts of it. However, this could not stop the infection in Samus' central nervous system, until the scientists finally thought of using Metroid cells as a vaccine to battle the parasites. This cleared the parasites out of Samus' body and suit, but the Metroid cells also fused with them, changing her appearance and giving her the inherent Metroid strengths (and weaknesses).