I am in between jobs and elected to use COBRA from my prior job until I find another job with health insurance. Let's say I am diagnosed with a catastrophic illness such as cancer while on COBRA? The maximum length of time one can be on COBRA is 16 months. IF I found another job that has health insurance, can I be denied or can the care for cancer treatments be excluded as a pre-existing condition by the new employer's health benefits.

It will depend on your new employers insurance, most have open enrollment so no one is turned down and have to be insurnaed. That is why group insurance from a job is great.

But many small business don't have that and their policies are underwritten, and you can be turned down.

And of course all private insurance will either exclude any cancer coverage or not insure you at all.