Very thorough on all the reasons chickens might sneeze and when we should be concerned about it. Great links for reading further if one wants to as well!
This is a well written, well structured article on its topic. It contains much sound advice, especially the points that "It's probably fine if a chicken sneezes but then continues with what it was doing afterward", and "You could make the situation worse by choosing the wrong treatment". It might make clearer that chickens can, entirely on their own, recover and gain partial or total immunity through exposure to some of these bugs; and that antibiotics will kill off the good bacteria as well as the bad bacteria in any bird so treated, so antibiotics should be a treatment of last resort, not first.
Good overview of respiratory diseases that can cause sneezing. I would add, though, that even though you can treat the symptoms for these diseases a lot of them cannot be cured. This means that the chicken still has the disease after symptoms subside and will pass it to any chickens it comes into contact with.
For that reason, unless you have testing done to see what disease the bird has (a lot of them have similar symptoms so without testing you can't really tell) the chicken should never be introduced to new birds, and the only way to get rid of the disease is to either cull and start over, or wait for the flock to naturally die off, and then start again.
