It is an unfortunate truth that chickens, just like every other form of livestock, need to be wormed. There are a lot of reasons to do this, and different methods available.
Chickens, because they forage off the ground, can easily pick up a worm infestation. In fact, it's almost inevitable. The most common source of infestation is ascarid worms, also known as large roundworms. Threadworms, tapeworms, and gapeworms are all also frequently present.