I am in the depths, fell down a hole and died to a curse when three frog things pounced. Is there a way to block the fog, or do you just have to kill them before they burp it up?
Fog is unblockable, those frogs are nasty, I got so scared the first time I feel down that hole (there are several btw).
I don't know if they are accessible at that time, but Paladin armor and Cursebite ring helps out A LOT! Also, humanity and being in human form improves your curse defence (meaning that the meter takes longer to fill up).
They have two methods of fogging you:
Either they stay stationary and you can clearly see that they are inhaling, in order to exhale their weird fog. This is the best time to attack them, attack them from the side while they are charging up their attack.
I don't think that they will trace you once they started the animation (that is, it will face the same direction during the whole animation).
Alternately, they do a nasty jumping attack, which has a somewhat longer range. It can be kind of tricky in the way that they kind of spit out a projectile of fog, which expands around it's point of origin. Just take note where their projectile landed, and move away.
Mobility is the key here.
Remember, it's always easier with one monster at a time.
If they got you, that means that you are cursed, and that means that now is a great time to farm the frogs (I think they are called Basilisk), since getting cursed a second time won't do anything (unless you lifted the curse status, which you can do by going to the tower where you rang the first bell, and talk to the black clad NPC standing close the ladder leading up, residing in the room you enter directly after defeating the Gargoyles and buying a curse stone from him, OR by buying it (curse stone) from the dead lady behind the grates in the aqueduct between Lower Undead Burg and Firelink Shrine). Find the bonfire in the Sewers, and then go down the stairs to the right, then climb down the ladder into the cage (inside the same room where the giant rat was), go out of the cage and walk down the stairs and then go left, trailing the waterway. If you go the right way, you should be attacked by some giant rats before you find the frogs.
Good luck, and don't let them get to you.
In addition to Apeforce's answer, there is a ring in the new londo ruins at the very end where the ladder to the top of the church is. On the bottom floor behind the building off to a ledge there is an item. A ghost ambushes you when you try to take it but the ring gives you around 100+ curse resistance making you basically immune to curse while wearing it. I have personally basked in the awful curse gas of the frogs and had nothing happen to me using that ring.
In case you don't know, to kill ghosts you need to be either A: Actually cursed or B: Use a transient curse item (You get three at the start of new londo from a corpse). After that you're free to kill ghosts who drop more of the item.
It's a fairly hazardous journey but once you know the path and all the ghost ambushes, you can easily grab the ring and enjoy immunity from curse.