To systematize the answers:
All separate sources of health regeneration and degeneration stack, however, the end result is capped at +10 regen / -10 regen.
Regarding the sources, it should be noted that you can only have one of the same effect on you. You either are bleeding or are not bleeding, you cannot bleed twice. Same applies to other effects. You can only have a single instance of a hex on you at the same time.
For a detailed example:
A character becomes diseased, then gets poisoned, hit with an additional poisoned attack, gets hexed with conjure phantasm, gets a Healing breeze enchantment from a friendly monk and then gets hexed with conjure phantasm again and is set on fire (burning) (not unlikely if the character is a tank

).
-4: The character would first suffer the -4 degen from disease.
-8: When poisoned, he would suffer an additional -4 from poison, making a total of -8 degen.
-10 (-13): The conjure phantasm hex would make the total degeneraion -13, however, this is capped at -10.
-5: The healing breeze adds +8 health regeneration, which would be deducted from the total of -13 (even though it only caused -10 in effect) for a -5 degen.
-5: The second Conjure phantasm hex on the character cannot coexist with the previous one.
-10 (-12): Burning adds an additional -7 degen, resulting in -12 degeneration, which again is limited to a maximum of -10.
In practice, health degeneration is not considered a good source of damage due to this limit, as the limit lowers the maximum Damage Per Second to 20DPS, which would take 20 seconds to kill a 400 HP moster. Usually, a character will be able to inflict much more damage using direct damage skills.
Edit: forgot Disease is -4 degen, not -3, requiring a fix to the calculations.