Several different kinds of parasitic worms can be passed from cats to humans via contact with infected animals or contaminated environments. Handling cat feces, such as when cleaning the litter box, is one frequent route of cross infection. Some parasites can also survive in contaminated water or soil.
Hookworms are common parasites that preferentially infect cats but can also live inside humans. Two species, Ancylostoma braziliense and Ancylostoma caninum, are very common in the southern United States and are known to infest cats, dogs and humans. They are most often caught via contact with wet soil that has been used by cats or other pets for laying their feces.
Several other similar species also infect both cats and humans. Tapeworms are known to cross infect from cats to humans if given the opportunity. The two most common in cats are Dipylidium caninum and Echinococcus multilocularis. E. multilocularis is transmitted to cats via the consumption of wild rodents and can infect humans if the worm's eggs are accidentally ingested.