wolfgang is incorrect. The same thing happened to me once, I was afk for a minute and the computer screen went back to the list. I entered back into the game and as soon as I entered my health randomly jumped down from 100 to 23. I have a Windows 7 computer with no lag at all and my health randomly jumped down. This is a bug that will most likely be fixed in a later update.

Taking 40-80hp damage. Also: teleporting 5 feet infront of me when I'm standing on top of a hill = instadeath... YEAY.

FIX THIS. SRSLY.

It's not intended

By spending 5 minutes and reading through other posts I found other people with your issue. It seems its not a bug or glitch, but an intended part of the game. If you have a bad internet connection or connection to the server (aim for the lowest ping, I try to stay under 100) then your character will start to lag, and to others, it will appear as if you are teleporting rather than walking. So that people do not induce lag purposefully and use the teleports to their advantage, you take damage when you lag.

Possible fixes are a better computer if it can't handle the game, playing server with the lowest ping possible, and obtaining a better internet connection.

Often I'm just minding my own business, and then my screen freezes and I take like.. 1-25 damage, just randomly. Even when there are absolutely no enemies around - sometimes even just after I spawn. It's so annoying because I am a builder; I gather blocks alot, so filling up the 100 block bag takes a while and I get some damage then, and then it takes abit to get to the place where I want to build depending on the terrain, by that time I've died 3x. Even when I do get to the place I want to build, I am only able to place 5-10 blocks before I die.

How the heck do I fix this?