Use醫用香皂to heal it(about9,990 NP).
Or go to the faerieland healing springs,but go to this website first:
http://www.neopets.com/faerieland/tdmbgpop.phtml
Click on the picture once,if you click it before you go to the faerieland healing springs,
there is a bigger chance to heal your pet from尼奧肺炎(or any other dieseases)