I assume you have a bit of a flea problem? As with anything prevention is better than cure.

I treat all my animals with drops - you put them on the back of their neck.

You need to firstly eradicate all the fleas - this means treating all your animals - seek vet advise/ products if necessary. You then need to fumigate your house with a good flea product - I am talking carpets, soft furnishings etc.

Then give your house a thorough vac (this is good for getting up all the flea eggs too).

Then once all above you need to follow a flea maintenance prevention programme.

As for making your animal ill - fleas can. They can cause a lot of irritation, anaemia and skin conditions.

HTH