>>723866894
Pahologic 2 is a remake of the first one, not a sequel.
But it's a closer to the way movies do remakes in a sense that it's not just a port to a new engine but many and many things are reworked including some characters.
However original game had 3 (2,5) playable campaigns with 3 different protagonists while P2 only covers one campaign. P3 covers another campaign and who knows when we will see the final one.
Also it's worth mentioning that while in P1 all three campaigns had almost identical gameplay with differences only in the story, P3 goes for something new and fresh in the way its played.

So answering to your question: you don't have to play P1 but it will be a good idea to play P2 since P2 and P3 happen at the same time.