Worms are highly infectious and are picked up by your cat from the ground, through the skin or via its mother as a kitten is nursing.
Once infected your cat's health will be affected in some way, especially where kittens are concerned.
Roundworms and hookworms can also affect people, particularly children, with serious consequences.
While treatment kills worms present in the intestine of your cat at the time of treatment it does not protect against future re-infestations. Once is not enough and regular worming is essential. Kittens are wormed every two weeks until they are three months of age, then monthly until six months of age. Adult cats are wormed every three months for the rest of their life.