Can fleas be spread from human to human?

Yes, this happens all the time. Fleas typically prefer to live on other mammals like dogs and cats because they can hide in the fur coat easier. However, if a human is the only meal they can find, fleas will happily feed on humans with no problem.
A flea would much prefer to live off of the blood of cats, dogs, or other animals, than they would that of a human. It is extremely unlikely for a human to be infested with fleas, but singular flea-bites are not uncommon.