Just like in the real world, on Neopia, Neopets can get sick. There are over thirty different illnesses that your Neopet(s) can get. Some are randomly occurring illnesses, like Sneezles. Others, like the NeoFlu can be "won" from games like the Wheel of Excitement. Still others, like the Hoochie Coochies, can be the result of eating certain poisonous foods or getting hit with a poisonous item in the Battledome.
Some illnesses are easy and relatively inexpensive to cure. Simply go to the Neopian Hospital and find out what the cure is, and a few hundred Neopoints later, you're pet may be cured! Other illnesses can cost over 25,000 NP to cure. At this point you've got three choices - you can feed you Neopet a Medicinal Plant, which *may* eventually cure it. Or, you can visit the Healing Springs every 30 minutes. Sometimes the Water Faerie there will cure your Neopet. Lastly, you can visit the Wheel of Knowledge, and for 500 NP, your Neopet might be healed by a Water Faerie.
Illnesses are bad for many reasons. Your Neopets happiness level drops immediately to about the half-way point and will not rise until your pet is cured. Also, your Neopet cannot fight in the Battledome if it is sick. And, f you're Neopet stays sick for too long, it's happiness level will drop to the lowest level and one day it might be so sad that it literally turns blue in color, a condition which will not go away after your Neopet is healed unless you paint it a different color or use a Morphing/Transmogrification Potion on it. Lastly, when you're Neopet is not happy, you are much less likely to get good Random Events while exploring the Neopets site.