Most of what we do — the websites we visit, the places we go, the TV shows we watch, the products we buy — has become fair game for advertisers. Now, thanks to internet-connected devices in the home like smart thermometers, ads we see may be determined by something even more personal: our health.

This flu season, Clorox paid to license information from Kinsa, a tech start-up that sells internet-connected thermometers that are a far cry from the kind once made with mercury and glass. The thermometers sync up with a smartphone app that allows consumers to track their fevers and symptoms, making it especially attractive to parents of young children.

The data showed Clorox which ZIP codes around the country had increases in fevers. The company then directed more ads to those areas, assuming that households there may be in the market for products like its disinfecting wipes. The Centers for Disease Control and Prevention recommends disinfecting surfaces to help prevent the flu or its spread.

Kinsa, a San Francisco company that has raised about $29 million from venture capitalists like Kleiner Perkins since it was founded in 2012, says its thermometers are in more than 500,000 American households. It has promoted the usefulness of its “illness data,” which it says is aggregated and contains no identifying personal information before being passed along to other companies.