Wearables aren’t just good at generating reams of personalized data. Wearables also may be able to provide alerts when people are getting sick, giving doctors a head start on making a diagnosis and determining a course of treatment, say researchers at Stanford Medicine.

Wearable sensors that monitor heart rate, activity levels, skin temperature and other variables can reveal a lot about what is going on inside a person, including the onset of infection, inflammation and insulin resistance, according to the study by the Stanford University School of Medicine.

Over several months last year Stanford medical researchers collected nearly 2 billion measurements from 60 people, including continuous data from each participant’s wearable biosensor devices and periodic data from laboratory tests of their blood chemistry, genetic makeup and other measurements.

Participants wore up to seven wearables or web-enabled biosensors that collected more than 250,000 measurements a day. Researchers collected data on weight, heart rate, oxygen in the blood and skin temperature. Study participants also used wearables to collect data on sleep, steps, calories burned each day and such cardiovascular activities as walking, biking or running.

The data was then compiled to give each participant a health and wellness baseline, says Stanford professor and chair of genetics Michael Snyder. Already, consumers have purchased millions of wearable devices, including more than 50 million web-connected “smart” watches and 20 million other fitness monitors, says Stanford Medicine. Most monitors are used to track activity, but they could easily be adjusted to more directly track health measures, Snyder says. “We have more sensors on our cars than we have on human beings,” he says.

Stanford Medicine conducted the wearable research as part of a broader initiative to study precision medicine, an emerging approach for disease treatment and prevention that takes into account individual variability in genes, environment and lifestyle, according to the National Institutes of Health.

The wearables research revealed that a constant stream of data when measured against an individual’s baseline level of health and wellness activity can be an accurate indicator of when a person is sick and the root cause. For example, in several participants, above-average readings for heart rate and skin temperature correlated with increased levels of C-reactive protein in blood tests, the study says.

C-reactive protein is an immune system marker for inflammation that is often indicative of infection, autoimmune diseases, developing cardiovascular disease or even cancer, Snyder says.

The wearable devices could also help distinguish participants with insulin resistance, a precursor of Type 2 diabetes. Of 20 participants who received glucose tests, 12 were resistant to insulin. Using the wearables data, Stanford researchers designed and tested an algorithm combining each participant’s daily steps, daytime heart rate and the difference between day and night heart rate. The algorithm was able to predict from that data which individuals in the study were likely to be insulin-resistant.

The study concluded that a baseline range of values and mobile health data collected from an individual makes it is possible to monitor environmental conditions, illness or other factors that affect health. This monitoring and measuring capability then can help doctors pick up on of change in a person’s health, and reach a diagnosis more quickly.

As a personal example on a long flight to Norway for a family vacation last year, Snyder, who also took part in the study, noticed changes in his heart rate and blood oxygen levels along with symptoms he thought might be indicative of Lyme disease. Later tests revealed he did have Lyme disease. “Wearables helped make the initial diagnosis,” he says.

With a precision health approach, every person could know his or her normal baseline for dozens of measures, Snyder says. “Automatic data analysis could spot patterns of outlier data points and flag the onset of ill health, providing an opportunity for intervention, prevention or cure,” he says.