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.