It's happened to every DM, and now it's happening to me.
The party of regulars, who had been playing together for about a year and a half, has lost one player (not PC) and is now gaining a new replacement. The PC's have reached 4th or 5th level, and now I'm faced with the question of how to bring in the new guy.
Introducing the character in a way that the PC's will find realistic isn't the problem; if I can't come up with a rationale for that, I might as well hang up my dice. I'm speaking here about the starting level for the new PC. I've gone for 1st as a matter of policy, and I'll tell you why.
The DMG states that it is acceptable to bring in new PC's at levels higher than 1st, but it definitely hedges and I don't think it's absolutely necessary, given the levels of the other PC's. The combat encounters, while very potentially deadly (several characters have been sent into negative h.p.'s in the last couple of sessions), don't really involve a lot of single-shot big h.p. soakers so much as tactical situations that pile up a lot of damage quickly, but incrementally. (Meaning, the 1st level PC might go down below 0 h.p., but he won't be taking 40 points of damage in a single shot for an instant kill.)
I should also point out that I have no problems with killing characters. It's part of the game. If characters don't die, then there's no real challenge, no real tension, and no real excitement. So if the new guy happens to go below the magic -10 h.p. threshold, he'll roll up a new one and hopefully that one will do better. Hopefully, though, the other PC's will step in to help prevent that from happening.
1st level characters in a 5th level game also have the advantage that they will advance very quickly, relatively speaking. They won't be killing rats and orcs, they'll be going after wererats and ogres (well, not literally). The treasure will be commensurate to the monsters, and the x.p. will thus be higher. He may always be at a deficit in total x.p., but the system is designed so that characters slow down in level advancement as they get higher, so eventually the new PC will reach level parity. Assuming he survives.