Toxocariasis is an illness caused by roundworms. These roundworms most often infect animals like dogs, cats, or foxes. The worms live inside of the animal's bowels. They produce eggs which are passed on through the animal's stool. The eggs can contaminate the dirt for a few years. You may come in contact with the eggs by accidentally eating small amounts of contaminated dirt. Then, the eggs grow inside of your body. They can travel to many parts of your body and cause problems.