Four days after our daughter was born this October, we got a call from the hospital — they wanted us to take her back in right away to rerun one of the newborn screening tests. It turns out her initial test indicated that she had practically no T cells, a key part of the immune system. While every doctor assured us that the test was very likely a false positive, they wanted us to avoid giving her live virus vaccinations while we waited for a retest.

I don’t want to keep you in suspense: The retest two weeks later learned that it was indeed a false positive, and she was totally fine. But for an anxious new parent, two weeks is a very long time, and while we waited for an answer, I spent a lot of time learning more than you’d ever want to know about the US newborn screening program. It’s one of our health care system’s most impressive accomplishments, and something parents usually never have to think about — unless something goes terribly wrong.

The extraordinary balancing act of screening for rare diseases

When a baby is born in California, where my family lives, nurses take a tiny blood sample from a foot prick and send it on to a lab. There, that spot of dried blood is tested for more than 80 serious but treatable genetic disorders. (Most states have a similar system, but the exact details of testing vary by state.) One of those is SCID, or severe combined immunodeficiency, which you may have heard of as “bubble boy” disease. SCID can be inherited, but it is often the result of a genetic mutation, and it affects about 1 in 58,000 babies.

© Vox