Well its not really proven and set in stone but just think about it, how many presidents or their family members have ever died or even been diagnosed with cancer. What about governors? No? Senators? Still no as much as I can come across.
Why don't we reveal it to the people? Well, the government won't let it happen. When Polio was cured to many people researching it lost their jobs. Also, think of all the companies and charities investing massive loads of cash going into it and they all pay taxes.
Maybe I'm wrong on all this, who knows? I'm just fourteen and thinking, not a conspiracy theorist or even a person who really knows that much about the government.