There is no information available to indicate that Johnson's wife "apparently didn't follow precautions", or that the virus absolutely cannot be transmitted to those who are following recommended guidelines. There is no way to positively conclude that Jimmie Johnson always did "follow precautions", nor evidence that he did not, to my knowledge. In Johnson's public statements, he describes both he and his wife as having been as careful as they could be.
There has been no determination made that Johnson's wife was infected with the virus before he was, or vice versa. They do not know. According to news reports, he tested positive in an antibody test "earlier this year", while she tested negative. Antibody tests have relatively high false positive rates (~20%), so again, nothing is certain.
Jimmie Johnson talked with the media Saturday morning, saying he has many more questions than answers after testing positive for COVID-19.
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