Had gathered a bundle of sticks - For the purpose of making a fire.
There came a viper - A poisonous serpent. See the notes on Matthew 3:7. The viper was doubtless in the bundle of sticks or limbs of trees which Paul had gathered, but was concealed, and was torpid. But when the bundle was laid on the fire, the viper became warmed by the heat, and came out and fastened on the hand of Paul.
And fastened on his hand - καθῆψεν kathēpsen. This word properly means to join oneself to; to touch; to adhere to. It might have been by coiling around his hand and arm, or by fastening its fangs in his hand. It is not expressly affirmed that Paul was bitten by the viper, yet it is evidently implied; and it is wholly incredible that a viper, unless miraculously prevented, should fasten himself to the hand without biting.