Half-Life: Particle Fusion is a Half-Life modification.

"5 hours, or was it 5 days? I couldn't tell while I was blacked out, all I could feel was this massive pain in my head at all times, no matter if I tried to sleep, it would still be there... and if i tried to open my eyes, all I would see is black. I truly was going through hell... Then out of nowhere, the pain got sharper for a second, then stopped. I opened my eyes... my god! I could see! It seems Walter had saved me, he told me that I had been latched onto the head by one of those "headcrabs". I then was told that I, so far had only blacked out for 5 hours. Lucky me, if I had been gone longer, I would have turned into one of those ugly zombies that the "headcrab" concocts. Then the last thing he told me was... "Be careful out there, and please stay away from personnel as much as you can... as we don't know the effects of what happened to you..." I picked up my silver beretta, and started my traverse through this hellhole named Black Mesa..."