hi, although my cat had a Program injection a couple of months ago, we've seen one live flea and a couple of dead ones. I understand this is how it works - fleas can still land on him but they die when they bite.
But he also seems just a little under the weather, not ill, just not himself. Can fleas make him feel like this?
A large infestation can make cats ill. There are a whole host of other reasons that your cat could be under the weather I would suggest a trip to the vet. Program does not kill live fleas it sterilises them to prevent further breeding in your house.