I have a 11 week old chick that I witnessed I believe vomit earlier. So I began a search of what could be wrong. Decided to look into symptoms of marek's disease. I found this page and amazingly enough it is in fairly easy terms to understand. Just thought I would share this with you.
http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf