Humans become infected with roundworms when they ingest infective eggs from the soil or from their hands or another object. Large numbers of the eggs can accumulate in the soil where dogs and cats are allowed to defecate. The eggs are sticky, and can collect on the hands and under the fingernails of people. Children, and others who may not have good hygiene, are most prone to becoming infected.

People become infected with hookworms when the hookworm larvae in the ground penetrate through the skin. Persons who have contact with the ground, especially for long periods of time, such as plumbers or electricians, and sunbathers, especially those lying on wet sand or ground, are at increased risk.

Roundworm eggs and hookworms eggs and larvae need to be in the environment approximately two weeks, before becoming infective, so direct contact with an infected animal generally does not result in transmission. However, young puppies may continually contaminate their entire litter area. Adults and children who handle the bitch or puppies or who clean the area may be especially at risk.

If a human ingests roundworm eggs, the subsequent larvae can migrate through the person's tissues (this condition is called "visceral larva migrans") or eye ("ocular larva migrans"). Hookworm larvae migrate through the skin and cause a disease known as "cutaneous larval migrans."

To prevent human infection, good hygiene is extremely important. Teach children, especially, to wash their hands after playing and before eating. Do not let children play in areas where dogs or cats may have defecated. Do not allow cats to use sandboxes or the garden as litter boxes. Worm your pets as recommended, keep the environment clean, and control rodent populations.