I agree with Cuwiar - except to say that, if indeed she and the others have severe worm infestation it would be better to get the Flubenvet powder and make up a very palatable mix with pellets + sunflower seeds + dried mealworms if possible, maybe soaking the mixture with water to make it damp and crumbly as this is more attractive to most chickens and you'll be sure to get enough into her/them. You mix the Flubenvet according to the instructions with the stated dose in a kilo of dry food. Add enough cod liver oil, or vegetable oil will do, to the pellets, enough to coat them lightly when well stirred, so the pellets are coated with the oil and the Flubenvet powder will stick to them evenly when you mix it in. You need to give a 7-day course and during this time you shouldn't give anything to eat except the treated pellets, to ensure they don't fill up on other untreated foods and not get their full dose according to appetite.
The other symptoms she is showing may be due to something else, because if a bird has a heavy worm infestation her immune system will be lowered and she will be more likely to go down with another problem as well. Your vet would advise you about this.
Vermex is quite useful as a toner for the gut and it claims to improve a chicken's resistance to worms, but does not eradicate them once they are present. You need to give Flubenvet twice a year to all your birds, every 6 months, even if they don't show symptoms, as once they get to the stage where worms are obviously pulling them down the infestation will be very heavy and affecting them badly. They get worm eggs from pecking on ground where the eggs have passed through the droppings of other chickens, or else by eating earthworms or slugs which are hosts to several types of chicken worms. (So don't give earthworms as a treat, however much your birds enjoy them!) The eggs hatch in the chicken's gut, and the resultant worms take nourishment from her so she gradually goes downhill and weakens. The worms lay new eggs which pass out in her droppings to continue the cycle. Another good reason for keeping their runs as clean as possible by regular poo picking!
If your bird was mine, I would definitely get them all wormed, especially since you don't know much about where they came from or whether they had ever been wormed previously. Worms may not be the only cause of their problems but are most probably contributing. If there is also a respiratory infection you'll need a trip to the vet for some antibiotics, and when you're there you can check on the other things you've been advised to find about. Good luck, and keep in touch to let us know how you get on.
PS Read more about Flubenvet on the Forum at http://poultrykeeper.com/poultry-medication/poultry-medication/flubenvet-used-for-worming.html