With iPhones and other mobile devices in the hands of more people than ever, the stakes couldn't be higher. Tech giants such as Apple, Amazon.com Inc., Microsoft Corp. and Google have to figure out, now that they are among the biggest companies in the world with some of the world's most ubiquitous brands, how to get even bigger.

The business of keeping people well is a logical frontier. Health care accounts for about 18 percent of U.S. economic output and still often relies on antiquated tools like fax machines, making it an enticing opening for tech behemoths looking for new terrain to conquer.

Already, the companies are in deep. Apple is tracking vital signs. Amazon bought its way into the pharmacy business while joining with two powerful partners to remake worker health coverage. Google parent Alphabet Inc. wants to help make you live longer.

There are reasons for skepticism. Apple's new pulse-monitoring tools could trigger false positives and panics, some analysts worry. Amazon and its partners hired a famous surgeon CEO for their worker health plan, but building it out could take years.

Still, the companies are moving ahead at breakneck speed. It can be hard to keep track of developments. Here's where things stand:

Amazon has its tentacles everywhere. It's working on the employee-health venture with JPMorgan Chase & Co. and Warren Buffett's Berkshire Hathaway Inc. Earlier this year, it paid $1 billion for online pharmacy PillPack after months of speculation about its plans to shake up the prescription-drug business.

Meanwhile, hospitals, doctors and dentists' offices have helped propel Amazon Business to $10 billion in annual sales. The unit sells everything from latex gloves and patient bed liners to specialized dental tools and medical devices. Customers include more than half of the 100 biggest hospital networks in the country, the company said Wednesday.

Amazon Web Services, the cloud-computing division, offers tools that can be used to move from paper to electronic medical records and help genomics researchers store and analyze vast amounts of data. Cloud computing lets customers ramp up processing power for trials and modeling only when they need it, allowing them to conduct tests more cost-effectively.

One, called Calico, has a team of biologists, doctors and drug developers that are focused on understanding the process of aging, with the very Silicon Valley goal of extending life spans. The second, Verily, is working on new ways to collect and use health data, such as a contact lens that monitors glucose levels. DeepMind, Google's main artificial-intelligence company, also has a significant health-care division.

Additionally, Google has special cloud storage, email and file-sharing products that are HIPAA-compliant. Google Fit is a suite of tools for Android phones that developers can use to build health-tracking apps, just like Apple Health. And don't forget the core search product. The company has been working to improve “Dr. Google” ever since people typed “what does a heart attack feel like?” into their browsers. Google now has a special team of doctors who help make sure results for common questions offer accurate information.