Billionaire Mark Zuckerberg thinks we can cure, prevent or manage all diseases by the end of the century. And he and his wife Dr. Priscilla Chan announced Wednesday that they’ll give $3 billion over the next decade toward the effort. Another billionaire, Bill Gates, was there for that announcement. The Bill and Melinda Gates Foundation has given more than $10 billion to fight AIDS, malaria, and tuberculosis. Earlier this year, fellow Facebook co-founder Sean Parker announced he’d give $250 million to find a cure for cancer. So what happens when billionaires’ private foundations get into the “business” of eradicating disease?