At Birth

• If the mother cat (called a queen) has not been regularly wormed, then she will have intestinal worms such as roundworms or hookworms. As the kittens develop in the queen's body, microscopic worms migrate from the queen's body into the kitten's. Thus, the kittens will be born with intestinal worms. ThePetCenter.com also notes that roundworms can also be transmitted to kittens via the queen's milk. But, if the queen is up to date on her de-worming schedule, the kittens should be safe from infestation. Still, according to "Cat Owner's Home Veterinary Handbook" (Delbert G. Carlson DVM, et al, 1999), it is possible for a queen to transmit roundworms through her milk, even if she has been routinely de-wormed. This means the kittens need to be de-wormed, no matter what the health of the queen.
  
  

From Parasites

• Mosquitoes and fleas can also transmit worms to cats, dogs and many other animals. When a mosquito bites an animal infected with heartworms, the heartworm larvae crawl into the mosquito's mouth parts and wait until the mosquito bites another animal, such as a cat. The larvae then crawl down the mouth parts and enter the cat's body. Tapeworms lay eggs, which are eaten by flea larvae, which eventually crawl onto a cat. When the cat grooms herself, she may swallow the flea larvae and, in turn, the tapeworm eggs. The tapeworm eggs then hatch in the cat's intestinal tract.
  
  

From Food

• Mice, rabbits and small rodents that cats like to hunt and eat also can become infected with worms. If the cat eats prey infected with tapeworms, the tapeworms will survive the digestive process to set up a new home in the cat's body. Spoiled food and garbage may also contain worms. When the cat eats the infected food, they also eat the worms.