You must give your neopet the antidote. A list of sicknesses and antidotes are on most neopet related sites or guilds so just look around.
Where can I buy the antidotes?
They can be bought from the Pharmacy located in Neopia's shops. They can also be bought from the Shop Wizard, traded on the Trading Post or through a friend.
How do I cure my neopet?
Your neopet can be cured by:
1. Going to your inventory, selecting your antidote and then giving it to your pet.
2. Going to the Healing Springs located in Faerie Land.
3. Buying mushrooms that fully heal you.
4. Feeding your neopet a 'Medicinal Plant'
5. Or using a guild hospital.
I cannot afford the antidote. What do I do?
Go to the Healing Springs. It is completely free.