From Valve Developer Community
buddha is a console command that ensures you never die by always forcing you to have at least 1 health point.
To enable and disable, type
buddha in the console. If it worked, it should say "Buddha Mode on/off..." in the console.
Bug: If the player has X HP, and gets dealt any damage between (X-0.001) and (X-0.500), they will die. Health is stored as an integer, and damage as a float. The game rounds damage taken to the nearest integer prior and subtracts it from the player's health value, which can bypass the 1 health point barrier.
god, a console command that makes you take no damage at all.