A recently published meta-analysis of parasitic infections in cats found that world-wide, cats that spend at least some time outside are more likely to be infected by parasites than cats that remain indoors. This often leads to transmission of parasites to owners, other domestic animals, and wildlife. Cats spending time outdoors were 2.77 times more likely to carry parasites.

More information on the study of parasites in indoor and outdoor cats is available from the New York Times. The original study can be read in Biology Letters.

Read more about parasitic infections in cats in the MSD Veterinary Manual.