Roundworms grow in the intestine laying thousands of eggs which pass out in the faeces (droppings). The eggs can survive for months or even years in the soil and need to lie in the environment for some time before they can infect another animal. They find their way into a new host either directly, (when eaten by a dog) or indirectly, (after being swallowed by a rodent which is then eaten by the dog). Immature worms also survive in the tissues of an infected dog. Immature worms can be passed from a mother to her puppies in the milk.