The most reliable way to play the original game on PC today is through official browser-based platforms:
If you specifically want the delisted PC application in your Steam library, you can still find remaining inventory from third-party retailers. These keys can be activated on Steam to trigger a download: Pandemic - Matt Leacock Games Pandemic: The Board Game Download PC Game
: You can play the full Pandemic online with others directly from your browser with no download required. The most reliable way to play the original