'Internal parasites' is a fairly polite way of saying worms.
Hookworms, whipworms, roundworms, tapeworms can all infect your
cat. In fact, most kittens are born with roundworms and
hookworms that are transmitted from their mothers. The mothers
pick them up from ingesting eggs found in soil. Treatment for
these should accompany your kitten's first vaccination. Fleas
are the major transmitters of tapeworms. The worms get into a
cat's system when the cat ingests the flea in the process of
grooming. You can see signs of these parasites in your cat's
stool. They look like tiny grains of rice. If your cat has one
or more of these worms, symptoms can include diarrhea, a dull
coat, or change in appetite. The cat may even develop a pot
belly. Internal parasites can be treated easily, but it's safer
to have your vet determine the type of worms present and
prescribe the correct medication.