Causes of Worms in Dogs

Hereditary: It’s very common for puppies to be born with roundworms, as the larvae are often transmitted from the mother just before birth or through nursing.Dogs may become infected with hookworms by one or all of four routes:

• Orally
• Through the skin
• Through the mother’s placenta before birth (in utero)
• Through the mother’s milk

Diet & Infections: Your pup can get roundworms if he eats roundworm eggs that come from another animal’s poop, or any food that is already infected. Eggs can also be carried by small animals such as rodents, earthworms, birds, and some insects. These animals are not the normal host, so in these species the egg never matures—but if a dog eats an infected animal, the egg can activate and grow into a roundworm once inside the dog.
  
Immunity: In cases of puppies, their immunity is comparatively week, thus they acquire maximum change of being diseased if they come in contact with any infectious substance.
   
Enviornment: Puppies and dogs can become infected with roundworms by accidentally ingesting eggs from the environment—the eggs can be present in soil or on plants or other objects.

Source: Medfly Editorial Team