Elizabeth Holmes, 30, is the youngest woman to become a self-made billionaire--and she's done so four times over. In 2003, as a Stanford undergrad, she founded Theranos, a Palo Alto company that's disrupting the business of blood testing, replacing the services provided by giants like Laboratory Corp. of America and Quest Diagnostics. "What we're about is the belief that access to affordable and real-time health information is a basic human right, and it's a civil right," she says .

Theranos has raised approximately $400 million, with a recent round valuing it at $9 billion--more than the market caps of any of her rivals. Holmes holds 50% of the stock, putting her net worth at $4.5 billion. With just a drop of blood Theranos can do the same tests that used to require vials of it, at lower cost. "It's one of those white-whale type of approaches, where if this were to become mainstream, then the entire diagnostic testing market would turn over," says Mike Cherny, the diagnostics analyst at International Strategy & Investment.

Right now Theranos is still small. Its testing centers have been installed at 21 Walgreen's clinics, with more on the way; it also has a partnership with Intermountain Healthcare.

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