To feathery, you need to be hysterical where fleas are concerned.
Fleas and tapeworms go hand in hand, you don't usually get one without the other and they are transmissable to people!
Most animals are allergic to some degree to flea bites and can get rashes, hot spots and all sorts of nasty, painful conditions as a result.
A heavy flea infestation can make an animal seriously sick and weak, in the very old, young or sick they can actually kill them.
Fleas can't survive on people, but that won't stop them biting you!
When you spot one flea on a pet, there are usually hundreds more busy breeding in your carpets, under your skirting boards etc..
They can stay there for up to a year without a host!
If you don't eradicate them ruthlessly and thoroughly they can sometimes breed and infest your house and get totally out of control.
Some of the threads made on here, netmums and pet forums by distressed parents talking about how fleas are jumping all over their kids, crawling on clothes are just horrifying.