Fleas need blood to survive, kinda like a tick.
The important thing that you should know is that not all fleas bit humans. Yea, that's right. Dog fleas for instance do not enjoy humans as a food source. But that is not to say that if your dog has fleas they will not bite you.
Cat fleas will bite anything. Cats, dogs, horses and humans. It makes no difference to them who they get their meal from.
Fleas are also very prolific. They lay eggs that hatch take nearly a year to hatch or they can hatch in just a couple of days depending on the temperature. The warmer the ambient air the faster they will hatch. You can go from a couple of fleas to thousands and thousands in just 3 weeks.
The flea larva feeds on flea dropping until that become adult fleas where become a nuisance to everything they touch.