I was concerned with not having the healing
spell before I went to the Serpent Temple,
but I just kept casting Heal on my characters
when they got down to yellow and I was fine.
The fountain in the library cures your poison
and after that it isn't far to the town.

I was pretty mad about getting poisoned there too, but I noticed that the serpent priests dropped poison cure potions about half the time, so it wasn't too bad. Nice of them to put plenty of them there

Yes I got that same problem, but I found answer from this forum.
Thanks to that person who told me that at priest or someone else can heal poisoning
by Cure Poison that belongs Vine spell category level 4.
Try to learn that quick so you won't have to heal your self so many times.