Traditional health-care companies might want to watch out. There are a few new players in town -- and they mean business.

That's the message from a new report released by PwC's Health Research Institute, or HRI. According to HRI, new entrants to the health-care market are "poised to shake up the industry, drawing billions of dollars in revenue from traditional health-care organizations."

Many of these newbies aren't start-up companies, though. Far from it. Some of the largest organizations from multiple industries are redefining what it means to be a health-care business. Here are three of the biggest companies mentioned in the HRI report that hope to claim their stake in the $2.8 trillion market. 

1. Apple (NASDAQ: AAPL)
It's no surprise Apple could be looking a taking a bite out of the health-care market. In several ways, the company has already revolutionized the industry without fully targeting the sector. Now Apple could be getting serious.

iWatch concept art by Todd Hamilton. Source: Toddham.com 

HRI mentioned that the Cupertino giant secured a patent last year for a "seamlessly embedded heart rate monitor" that could run on its iPhones. That's just the tip of the iceberg.

The New York Times reported that several high-level company executives met with representatives from the FDA in December to talk about mobile medical applications. This meeting, combined with Apple's hiring of some prominent health-care experts to its team, prompted extensive speculation that the company could be making a major push into mobile health care, possibly with a forthcoming iWatch. 

2. Google (NASDAQ: GOOG)
Speaking of extensive speculation, Google's creation of a new company called Calico has sparked its fair share of rumors. In September, Google CEO and co-founder Larry Page announced the formation of Calico to focus on "the challenge of aging and associated diseases."

Google put together a powerhouse team of genetic researchers headed by Arthur Levinson, former Genentech CEO. Exactly what they're working on remains something of a mystery. However, research conducted by one of Calico's latest hires provides a clue. 

Aging-research pioneer Dr. Cynthia Kenyon joined Google's effort as a consultant last November but came on board full time this month. She helped start a company in 1999 seeking to develop a so-called "fountain of youth" drug. Her research at the University of California, San Francisco, focused on identifying "genes, pathways, and drugs that can extend lifespan." 

3. Wal-Mart (NYSE: WMT)
Giant retailer Wal-Mart is taking a less exotic approach -- but one that could be a game-changer for the health-care industry. The company partnered with Kaiser Permanente in 2013 to establish micro-clinics in a couple of its California stores. The clinics feature diagnostic devices such as blood pressure cuffs with telemedicine connections to Kaiser Permanente clinical professionals.

While Wal-Mart monitors how this California experiment fares, it is moving forward on another front. SoloHealth's health screening stations are already in many Wal-Mart and Sam's Club stores. The stations allow shoppers to check their vision and blood pressure and complete health risk assessments. 

Marcus Osborne, Wal-Mart's Vice President of Health and Wellness Payer Relations, said "there is a new world order coming." What's in this new world order of health care? He noted his company is still exploring and learning. However, whatever new services that could emerge from these efforts, Osborne thinks, will be "heavily technology-enabled" and "dead simple to engage with." 

Watching out
These aren't the only giants of industry moving into the health-care world. HRI also noted that Ford, for example, is working on ways to embed chronic disease management into its cars. Ford executive Gary Strumolo stated that the big automaker is "dedicated to understanding the value of being able to connect to health and wellness-related services while driving."

Traditional health-care companies and providers should be watching out for these new competitors. HRI's research found that many consumers are ready to jump aboard new and more convenient models of care. Companies working on innovative new approaches for health care just might be the big winners for this nascent market.

Dr. Apple, Dr. Google, and Dr. Wal-Mart? It's not that far-fetched.