The bottom line is that mozzies are the major carriers. I have had painteds as mentioned in previous posts with mozzie bite, didn't go bad, went after a couple of weeks but looked uncomfortable for the bird. Just remember that not all mozzies carry the virus, they bite and cause discomfort to certain breeds especially canaries, which are susceptible to this particular virus hence the name "Canary Pox".. It is only if you have a bird that has the been infected with the virus or a mozzie that has had contact elsewhere that problems can occur.
In short, try and keep the mozzies away, no stagnant water etc, etc. Yet here I am looking at using mozzie wrigglers as live food. Go figure

Ken.