The proper way of saying "Human to Human" is Human to Human. EX. Can a worm pass from Human to Human? No.
Some humans are just cruel people. Avoid them and look for kinder people. They ARE out there.
Opinion. Perhaps this question assumes an unfair generalisation. It is part of the basic human character to be concerned about our fellow human beings. Stories regularly appear in the press about...
yes
Probably more prokaryote cells on or in humans than their own eukaryote cells.