In-store medical clinics like those at Walmart ? having established a beachhead with relatively healthy patients looking for convenient, low-cost care for simple problems ? are eyeing a bigger prize, the millions of Americans with costly illnesses such as diabetes and heart disease. Just as Walmart and other retailers shook up the prescription drug business by offering $4 generic drugs, the industry now aims to apply its negotiating and marketing clout to tackle problems that vex consumers and the health sector: unpredictable costs, a lack of primary care doctors and inefficient management of chronic illnesses, whose costs drive the majority of health care spending. "It's sad that the existing health care establishment has not figured out a way to make primary care affordable and accessible," says Jerry Avorn, a professor of medicine at Harvard. "We should not be surprised if someone outside of our world comes in and does it for us."