Reading the journal entries Clemens made aboard the AMERICA, there is one
note from the editors indicating the illness Clemens suffered from on this
trip was something he had apparently contracted in Oct. 1866 while
lecturing in Nevada.

I don’t see any indication in the journal entries written aboard the
AMERICA where Clemens exhibits fear of dying aboard the ship — the Dec. 22
entries appear fun filled with singing and playing leap frog — certainly
not a death ship atmosphere — and the midnight entry on that date is almost
euphoric describing the full moon and balmy atmosphere. The only death
Clemens apparently witnessed or knew of aboard the AMERICA was the child on
Christmas Eve.

No doubt that things changed aboard the SAN FRANCISCO and the editors
document seven deaths aboard that ship before reaching New York. I am not
sure where the count of 20 deaths on each vessel comes from.