It depends on the disease/ailment. Some problems I've dealt with include parasites, frostbite, respiratory diseases, injured combs/wattles, and Coccidiosis. I treat internal parasites with Sevin dust or Frontline, and internal parasites with the Worminator de-wormer. Frostbite can partially be remedied by keeping the bird warm (to prevent further damage), applying VetRX or Vick's Vapor Rub to the comb to stimulate blood flow, and massaging of the affected areas. But if the tissue is dead, then it will fall off no matter what you do.
Respiratory diseases can be tricky to treat. My favorite antibiotic is injectable Tylan50. The dosage is 1cc for large-fowl, .5ccs for bantams, injected into one side of the breast once daily for 5 days. While it won't cure viral diseases (no antibiotic will--they will only help prevent secondary infection), Tylan will effectively and quickly treat bacterial respiratory problems. I also give my sick birds vitamins/electrolytes in their water, and encourage them to eat plenty of nutritious feed to build up their strength. Injured combs/wattles usually heal fine on their own, but applying antibiotic ointment will help prevent infection. Coccidiosis can be a major problem in chicks/young birds if not treated; fortunately, treating with 9.6% Corid liquid or 20% Corid powder usually clears it up. The dosage I use is 2 teaspoons/gal. for the liquid, and 1.5 teaspoons/gal for the powder, given for 5-7 days.
I've also had limited (kind of) experience with leg problems in birds. For an injured leg, rest and vitamins are the best cure. Weak legs can be caused by a vitamin deficiency, so I treat with vitamins (either regular poultry vitamins or polyvisol infant vitamins). And splayed legs in chicks can be corrected by tying/taping the legs into the right position.
Sorry for rambling. I wish you luck with your sick and injured birds!
