The 57-year-old writer and producer, who devised the hugely popular cartoon with Matt Groening and James L Brooks, was told by doctors he has colon cancer last year, but reveals the potentially deadly disease has spread to other vital organs, reported E! online.
"The diagnosis was - I was feeling bad for a couple (of) years, and I was diagnosing myself from television talk shows, so I was taking antidepressants and I was trying to get my testosterone levels up," Simon said.
"But then it turned out my primary cancer was colon cancer that had spread to my liver, kidneys, abdominal connective tissues and my lymph system and maybe my brain. They're not sure about that," he added.
"I was overwhelmed by all your kind tweets... I have great docs (sic), traditional and alternative, and I plan on beating this. Thanks, again."
Simon has also worked on hit US shows 'Taxi', 'Cheers' and 'Friends'.