There are ten in total in the lower sewers levels. They're encountered when you fall down through a trap on the ground (a hole, so watch your steps ; you should be warned that a rat will try to lure you through one of those traps by standing on it).
When you fall, you will be attacked by a group of those handsome devils, and they'll sneeze on you like it's winter in a frozen hell. Forget the shield, dual wield your weapon and deal with them carefully but quickly (they don't have physical attacks).
Alternatively go straight down the stairs from the bonfire. Down the ladder to your left, and down the stairs at the end of the hall. Take an immediate left and into the tunnel with 3 large rats, and 1 small rat. Around the corner is the first Basilisk, and the other 9 are ahead of you from there. This particular run is an easy way to farm for Eyes of Death and humanity.
Be aware that a much more bothersome kind of these monsters can be found in the Great Hollow (area accessible through the base of Blight Town). They are more resistant than their counterpart and should preferably be confronted one on one.
Another way to avoid being killed by Curse is to acquire curse resistance. Getting the Cursebite Ring from New Londo Ruins gives enough resistance to survive several simultaneous curse bursts. However, fighting through the ghosts is difficult if not impossible without a permanent method of damaging them. Curse resistance is also granted for every point of humanity showing on status screen. Consuming your spare humanity fragments or hunting more down may give you a little leeway with mistakes. Having a few Purging Stones on hand before you attack the frogs is advised.
If you get caught in a cloud, immediately roll away. They have very little hp and are easy to dispatch.
Easy to kill at range with one shot. Use Firebombs from the Undead Merchant Male at 50 souls a throw and the frogs give back 200 souls in return. The frogs burn really quickly and die even if not hit directly. (Firebombs have a nice Area of Effect) The player will require about 30-40 for all of The Depths to comfortably get through without much worry and a nice pocket of souls in return.
The Drake Sword can be quite effective with its two-handed shockwave (Heavy) attack.
A long thrusting weapon like the winged spear or the halberd can be quite effective at reaching the frogs when they're in a curse cloud.
Eyes of Death (rare)