Young pets are much more vulnerable to the ill effects of parasites than their parents. When a litter is born into a nest with fleas it provides the perfect conditions for the number of fleas to rapidly multiply. The presence of warm bodies in one location allows fleas to rapidly complete their lifecycle, ready to reinfect the occupants of the nest.
The small size of young puppies means that the loss of blood caused by fleas can cause life-threatening anaemia. The burden of fleas on young animals can overwhelm their immune systems making them more susceptible to other illnesses. Fleas can carry the eggs of tapeworms, when an egg-carrying flea is ingested, the tapeworm will hatch and attach themselves to the intestine lining. Intestinal worms pose a greater risk to puppies than to adult dogs. Symptoms include diarrhoea and a reduced ability to absorb nutrients from food, but in severe cases the number of worms can cause complete intestinal obstruction.