It can be bought from gamersgate: http://www.gamersgate.com/DD-PATH/pathologic
Myself; I gave up. Couldn't stand playing it after a while. Now, over a year later I'm giving it another try.
There's few games that can change one's view of gaming as a medium, Pathologic is one of them. Spend a few hours in it, and it will be remembered. Few games leave such a lasting impression.
This russian gem deserves to be mentioned.
It's not fun. It's so depressing I could barely stand playing it for long stretches. 10-15 mins was enough, 30mins was horrible.
The unique concept results in a game that *is* a depressive nightmare. It's unique in a way; what other game makes you feel like killing yourself - just to end your own suffering? It's absolutely brilliant, and a hell to survive through.
Interesting article about it: http://www.rockpapershotgun.com/2008/04/10/butchering-pathologic-part-1-the-body/