There are a few supplements for bird health that you can use when they're ill. These products give them energy to fight the illness. They can be vitamins or other substances like herbal ingredients.
For example, Morning Bird's THRIVE is full of high potency vitamins, minerals, amino acids, electrolytes and protein. These help boost the immune system and in turn the bird can fight off disease more readily.
These are products you probably want to keep on hand at all times. Remember, yes they cost money but it's still cheaper than going to the vet!