Thanks for writing! If you are not able to afford the medicine right away don't worry. Neopets won't get any sicker, and they won't die from the illness, so you have time to save the points for the cure. But while you save your Neopoints, make sure to start taking your pet to the Healing Springs located in Faerieland here several times each day for a chance to heal your pet for free!
Hope your pet feels better soon!