If a flea bites a human, does it usually itch or does the skin just get red?

Varies: The response to any insect bite varies with the individual. A typical response includes release of local chemicals to the saliva of the insect. These dilate local capillaries & create the bite mound which functions to block direct access of that saliva to the blood stream. These chemicals may trigger itching or redness.

Variable: There may be redness, itching or both.