What happens to your insurance if you have a lifelong disease, and you change insurance providers?

I was curious about how that would play through. Most insurance companies will not take you on as a customer if you have a pre-existing condition. (since it's hard to make money off say, a diabetic.) If your insurance is through your work and you change jobs, what happens? Since most of the time your insurance is dropped when you leave a job, i imagine it would be VERY tough for someone with AIDS, Cancer etc. to find another insurance provider.