I also have an illness I've had to explain to my children.
I've found that they aren't stupid or unobservant. Just explain things to them- be very honest. And don't answer any questions they didn't ask. In other words if they ask "Will you die?" You can tell them easily that you won't as long as you take care of yourself. It's honest, it's to the point.
If they ask, "Why are you sick?" You don't need to go into a full medical breakdown for it. A very simple, "Some people get sick with different things, and this is how I am sick..." will work.
The kids will appreciate the honestly more than you know. If you keep it mysterious, or too much information they will worry to themselves. If you tell them honestly, they will keep very optimistic but realistic views of it.
My children know that "Mommy is sick." When they are ready to ask me about it they learn, "Mommy has a headache." Only my oldest two know that I have a condition that mimics a brain tumor. Imagine trying to explain that to them... lol. They were worried everytime I had a headache until they learned that it wasn't a tumor and finally could understand what it is. Honesty works well with children. You don't want them to lie to you, so don't lie to them.