It'll be fine, I promise! Just like dogs and cats, chickens can pick up worms from their environment, wild animals, etc, it happens all the time. Most BYC folks worm relatively regularly as a precaution.
If you do some searching around here you'll find there are several deworming options. You can pick up most wormers from your local feed store, no reason to go to the vet.
I would worm all of your chickens, once one has them, most likely everyone will.
X2. I would absolutely deworm them all. Often once you actually start seeing worms or start seeing hens not acting up to snuff they may be carrying a heavy worm load. Internal parasites are very damaging and you want to keep a handle on it.
Wazine will kill only one species of worm, round worms. On the other hand if you use Valbazen it will take care of everything. I deworm twice a year. 1/2 cc for standard hens, 1/4 cc for bantams with a repeat in 10 days.