When I quit my job to run a company, I was very healthy. I just couldn't buy health insurance on the open market because I had a pre-existing condition.

This was prior to ACA, so I ended up paying through the nose for COBRA.

Pre-existing conditions aren't always debilitating. They can be things that, when treated appropriately, don't impact your ability to start a company at all: diabetes, amputation, cosmetic issues like severe hair loss, sleep apnea, amputation, ect, ect.