Parasites are very unhealthy for any animal. If left go they can weaken the immune system and increase the chances the animal will die from a secondary problem. Same goes whether they are internal or external parasites.
Apple cider vinegar (unpasturized or not) at 1-2 tablespoons per gallon of water. They is no withdrawl for eggs or meat with this. Not to mention it is all natural and works on all types of intestinal parasites on all different animals. It can be used several times per year or everyday however you desire. Vinegar changes the PH of the digestive tract, it does not kill the parasites, rather makes them want to leave and therefore does not carry the caution that other chemicals do. As someone else has mentioned there are a lot of other benefits for the bird by using it.
Without treatment you may see the parasites but not very often. If you do see them they are only the worms that have matured and are at the end of their lifecycle (dead/dying). There are still a lot inside breeding and feeding on everything the bird eats. Like the old saying about rats if you see 1 there are 1000. There are many different intestinal parasites some are microscopic like coccidia. This is the most common type for most species including chickens and the dewormers listed in another reply on here do not affect coccidia. You would need to use a product like Corid that is specifically designed for them and has an egg/meat withdrawl. If the chicken is highly overloaded it is possible for a worm to migrate from the cloaca into the reproductive tract. If this happens, chances are good you will loose that bird if not treated. Some parasites naturally migrate into the respiratory tract. These can cause great distress to the bird and destroy their lungs eventually killing them.
If you do treat with chemicals or vinegar it is important to repeat treatment several days after the intial treatment to ensure you limit the lifecycle. A lot of the treatments only affect parasites in certain stages. Doment eggs or immature larva may not be affected so you need to wait for them to come into the stage the treatment affects. Use strict caution if you believe the birds may be heavily loaded as medications (not vinegar) can cause a rapid die off of parasites and make the bird toxic therefore killing it.
Unlike someone posted, I believe you will never have worm free birds, whether you use chemicals or homeopathic treatment. Most parasite eggs are transmitted by insects. They may be short lived but it is a continuous cycle of egg ingestion (bug) and parasite worm death. They have various lifecycles but only lay eggs for a short period of time. If you do fecal analysis they check for eggs. If there are little to no worms in the reproductive stage, the fecal may say negative even though it should really say no evidence of parasites seen. This means there is not an overload of adult parasites and the bird should not be affected from them.
Check out the U of MD website about backyard poultry.
http://www.agnr.umd.edu/Extension/a...les/Raising Your Home Chicken Flock_FINAL.pdf