Well, not generally dangerous to humans, although they can be troublesome.
When fleas bite humans, they may develop an itching rash with small bumps that may bleed. Most flea bites require minimal treatment with a mild cortisone cream or an antibiotic ointment.
Infestations are a concern, however, and can cause prolonged discomfort, respiratory trouble and other symptoms.
Furthermore, fleas can transmit tapeworms and diseases to pets.