Separate birds who are ill. Treat external lesions with iodine or blukote, watch for signs of internal lesions, and if you find them, again separate the bird and swab the throat with listerine (the original icky tasting kind) at least twice daily, removing as much of the thick white/yellow coating as possible each time. It will bleed and be painful, but if it is not removed it will block the birds airway and throat. A bird who cannot breate or eat will not live.
Also, spray the yard and coop for mosquitoes to kill as many as possible, and nightly spray the coop with a mosquito repellant such as you would use on yourself. Mites can also pass fowl pox, so make sure to put poultry dust or sevin in the bedding.