Dr. Conrad joined Google X—the company’s research arm—in March 2013, and he has built a team of about 70-to-100 experts from fields including physiology, biochemistry, optics, imaging and molecular biology.

Other mass medical and genomics studies exist. But Baseline will amass a much larger and broader set of new data. The hope is that this will help researchers detect killers such as heart disease and cancer far earlier, pushing medicine more toward prevention rather than the treatment of illness.

“With any complex system, the notion has always been there to proactively address problems,” Dr. Conrad said. “That’s not revolutionary. We are just asking the question: If we really wanted to be proactive, what would we need to know? You need to know what the fixed, well-running thing should look like.”

The project won’t be restricted to specific diseases, and it will collect hundreds of different samples using a wide variety of new diagnostic tools.