Yes, ghouls can get sick based on their element.
Fire:
Fire's hardly ever get sick, unless they live in a colder region on Earth. During the winters months when it cools down, if a Fire gets too cold, they will come down with Hypothermia. Usually Fire's never die from it unless they were left out in a storm, but if they are taken care of properly, they will be fine.
Water:
They can get sick during any time of the year. If a Water goes over 48 hours without being submerged in water, they will become dehydrated. Although it is fairly easy to nurse them back to health, the chances of them dying are much higher then Fire's.
Quintessence:
The Quintessence are the ghouls least likely to ever get sick other then Air. Quintessence ghoul's hold the power of life in their magic. Quintessence ghoul's can only get sick if they use too much power, but even though they rarely get sick, when they do, it's too late.
Earth:
Earth's don't get sick easily either, but the chances are higher then Quintessence and Air. Earth's contain the energy of Earth and soil, so if they are distanced from the warmth of the soil for too long, they would get sick, which is why if it's a ghoul living on Earth in a colder region, it's best to always have a large greenhouse for your ghoul to spend the winter months in.
Air:
Air's never get sick, and if they do, it's very, very bad. When it's too hot or too cold for their Air element, they will die almost immediately, which is why they are the most vulnerable, but an Air dying from sickness is a 1 in a thousand chance.
Multi's:
Mutli's are most definitely different creatures. They can't really get sick. They have the ability of every element, but it's not like the real thing. They don't need the soil to keep them going, or water to keep them alive. The abilities balance themselves out, making it next to impossible for multi's to get sick.
Another words, ghouls can not contract human viruses.
