Illness is a problem that you will come across at some time during your poultry keeping.
Its a bit too late now but bringing in illness is a risk that you run everytime that you bring in new stock. If at all possible you should try and keep new comers in quarentine for a few weeks before putting them in with your eisting stock.
Its extremely difficult to diagnose long distance and almost as tricky when you have the birds in hand.
We've touched on this before, but vets are in the main not very knowledgble when it comes to the diseases of poultry. There are exceptions of course and I wish that one lived near to me.
This is what I'd do in the first instant.
I'd neck ( cull) anything that is really sick and looking as if its not going to pull through. I'd go to the vets and ask to be supplied with some Auromycin . This is a green powder that you give to the birds via their water. This is an antibiotic.
It wouldn't hurt to clean the pen out and disinfect it and perhaps a change of ground if at all possible.
Getting rid of the crawlies on your chickens, isn't going to stop their sniffles but it will make the birds feel more comfortable, which of course, can only be a good thing for them. So in the short term, a few squirts of a proprietary flea spray is going to sort this out.
As I say, diagnosis long distance is very tricky, but if the birds have a cold, then an injection of 1/2 cc of tylan 50 into the breast muscle of each bird is recommended and at the same time give a similar injection under the skin at the back of the neck. This should be done everyday for five days.
For flock treatment rather than individual, use the Auromycin in the water for 10 to 14 days.
I would also use a product called Vetorex which I get from the USA. This vapourises and helps clear the respiratory system. Because its unlikely that you'll be able to get your hands on this, if I was you I'd get some Vicks vapour rub and each night I'd rub a liittle blob in just beneath the chin and some under each wing just as they start to roost.
I hope that this does not seem to be too complicated. Its at times like these, that its a shame we are all spread out throughout the country.