Whoever first coined the phrase 'breeding like rabbits' clearly didn't know much about fleas! Where a wild female rabbit may give birth to 30-40 young in a year, a female flea can lay 30-40 eggs in just one day. 
 

Once on your pet, adult fleas take a blood meal (bite) and mate. Within 24 to 48 hours, the female starts laying her eggs. These eggs fall off the animal, wherever it goes in your house.
 

A typical female flea may lay 200 eggs over a period of five days. They'll hatch in a further 4-12 days, depending on the temperature and humidity.
 

So, two important facts here:
 

1. For every flea you can see on your cat or dog, there may be another 200 eggs around the house.
 

2. This is why experts the world over agree that the most effective flea control involves using 2 types of insecticide: one to kill adult fleas on the pet, and the other to prevent fleas reproducing.