Just an idea but maybe leprosy which made all your skin white,
wasn’t actually leprosy!
See here for example the comparison between leprosy and psoriasis
which makes your skin silvery.
https://www.healthline.com/health/psoriasis/leprosy-vs-psoriasis#treatments
I’ve had a search through a handful of medical websites, they seem to suggest
leprosy can show itself anything between 8 - 20 years from first coming
into contact and there is a milder strain and a more severe strain.
It causes patches of inflamed red skin, bumps, lumps and over time the
patches look paler than rest of the skin. I can’t find any reference though to
it covering the entire body - possibly due to how long it takes? Maybe
people with genuine leprosy lived with it for years and years and died of
infection (if they cut themselves and couldn’t feel it), long before their
entire body turned white.
Whereas people whose skin did all get pale or white which developed
quicker, had some other skin condition.
I know there is the story of Miriam but assuming it takes between 10-30
years for leprosy to show itself and get really bad assuming you live that
long, you can see instantly that it was definately an act of God.
According to the medical websites, leprosy also causes internal damage
to organs and tissues.
Another reason maybe why it was unlikely peopl with leprosy lived long
enough to become totally covered.
Remember they didn’t have the medical science we have today, but God gave
them instructions about all sorts of skin diseases. They had to show themselves
to the priest, who acted like pseudo doctors.