Image copyright Theranos Image caption Elizabeth Holmes is worth $4.5bn (£2.7bn)

Monitoring what's going on in your body has gripped California's Silicon Valley like a mania.

Enthusiasts wear two or three wrist bands to keep an eye on their blood pressure 24 hours a day.

They use sensors to tell them how many paces they have taken - the recommended daily rate is currently 10,000, I think. That's about five miles.

And up and down the valley, new companies are rushing to get a piece of the action. They are matching body measuring devices to the smartphone, to produce a torrent of data that may or may not be useful to doctors and specialists, if they have the time to deal with it. There are dozens of such entrepreneurial start-ups, maybe hundreds.

It is happening because only very recently have people become permanently connected to the internet in this always-on, display-rich way. Mobile technology is seemingly reordering our relationship with ourselves, as well as the outside world.

After you've liked or friended a person, you may as well include your own body in your digital network.

Self-made billionaire

The beginning was what I could do to make a change in the world Elizabeth Holmes

Meanwhile, on a corner of the Stanford University campus where Facebook once had its office, Elizabeth Holmes is working away at a health monitoring project that has already taken up 11 years of her life.

Her company Theranos is the antithesis of the digital healthcare gold rush, though it is not unconnected with it.

She is only 30, but she has been nursing this company all through her 20s. Only now is it breaking cover and making itself known.

Elizabeth Holmes has that unshakeable surety of purpose that is one of the hallmarks of a Silicon Valley entrepreneur.

She dresses in black like the late Steve Jobs of Apple. Like him, too, she generates a kind of force field of attention and self-confidence.

Image copyright Getty Images Image caption Unlike some rivals, Theranos' technology only requires a drop of blood

And she seems to have had it since at least the age of 19, when she dropped out of her engineering studies at Stanford University to found her corporation.

Theranos is as yet little known, but private investors have taken stakes that value it at an extraordinary $9bn (£5.4bn).

Ms Holmes still owns half the business, making her on paper (according to the magazine Forbes) the youngest woman ever to become a self-made billionaire.

And Theranos has attracted some striking believers. On one of the most star-studded boards of directors in the USA sit two former secretaries of state - Henry Kissinger and George Shultz - and a former defence secretary.

Theranos has what is on the face of it a straightforward purpose - to make blood tests simple, timely, unalarming, and cheap.

Elizabeth Holmes is convinced that she is on a big mission: "The beginning was what I could do to make a change in the world.

"To affect people's lives in a meaningful way."

'Difference to the world'

A huge number of medical diagnoses are based on blood tests - billions of tests a year in the US alone, costing tens of billions of dollars. Yet for many people they are unaffordable and invasive. There's a widespread fear of needles and testing.

Image copyright AP Image caption Theranos has some powerful shareholders

As a result, Ms Holmes says that in the US, nearly half the people do not get the tests that their doctors order for them.

Theranos has a upfront price list for more than 200 specified blood tests, seemingly far cheaper than established test companies, where the bill comes in after the tests.

Much smaller blood samples are used to do tests - little more than a drop. And then the blood is analysed fast in company facilities - automated labs shrouded in proprietary secrecy.

"We are handling such small amounts of blood that we needed to redevelop the chemistries and the analytical systems on which to run them," she says.

Interesting yes, but is affordable blood testing really a revolution in healthcare?

Elizabeth Holmes says it's about access to information: "We believe that when someone you love gets really, really sick, by the time you find out about it is usually too late to do something about it... a very painful experience."

Image copyright Reuters Image caption A growing number of us use mobile phone apps to monitor different aspects of our health

"If we could build a system that would help to change that, then we would make a difference in the world."

To that end, Theranos has recently launched an alliance with the largest drugstore chain in the USA.

Walgreen's has more than 8,500 stores all over the country, within five miles of a huge swathe of the US population. The stores have just started installing what are called Theranos Wellness Centers.

I dropped in on one a mile or so from the company's HQ in University Avenue, Palo Alto.

The experience was certainly straightforward - a welcoming staffer put a soothing warm wrapper on my finger, a pin prick which I hardly felt, and then a small phial of blood filled in a second.

The results were emailed back in 24 hours.

'Saving money'

To change the trajectory of early detection of illnesses, says Elizabeth Holmes, it is very important to be close to where people live.

For some years before this retail drug store rollout, Theranos has been making money by selling its services to big pharmaceutical companies.

For them, large-scale testing of drugs in development is an expensive, time-consuming and cumbersome process. It's the stage where costs soar in the development of a new drug.

Speeding up tests gets more information about the efficacy of a new drug back to base faster. Easy, frequent blood testing may enable drug companies to see the impact of adjusted drug dosage on their trial patients much faster.

Theranos has Europe in its sights too. The cost to patients of blood testing may be a hidden issue when a health service pays, but there is - says Elizabeth Holmes - the chance to save "an incredible amount of money".

And when people start taking a much closer interest in their health by having blood tests more frequently, then illnesses will be spotted earlier, she thinks.

Blood testing is a market worth billions in the US alone, dominated by big corporations such as Quest Diagnostics and LabCorp. The established companies are unlikely to let a newcomer such as Theranos gain market share by waiting to be undercut, if it can roll out its services as it intends.

Theranos is a highly ambitious company with a strikingly ambitious founder and a lot still to prove.

But healthcare is only just beginning to wake up to the implications of personalised medicine. And diagnosis - including blood tests - will be at the heart of some great big changes in the way we think about our health.

Peter Day is currently reporting from Silicon Valley - and its new obsession with digital healthcare - in Global Business on the BBC World Service.