Have you actually read any of the descriptions of how it works? Health data is not transmitted. No information ever leaves the device, with the sole exception of when you choose to notify other that you tested positive. That uploads enough material for all other users to determine if they were ever in the vicinity of your device - but no other information. The only thing that can be determined is whether at some point they were in the vicinity of a device that subsequently reported testing positive.

You might find the FAQ here [1] helpful, but there is also more technical info linked from that page also.

[1] https://www.apple.com/covid19/contacttracing