Roundworms are very long, spaghetti-like worms. Puppies and kittens contract roundworms in utero or from nursing. They can cause poor growth, a pot-bellied appearance, and diarrhea. In severe cases roundworms can cause blockage of the intestines.
Hookworms are a parasite that feeds on blood from the intestinal wall. Puppies and kittens can be born with hookworms and a heavy load of hookworms can make the puppy or kitten very anemic.
Whipworms cause a mucousy diarrhea. Whipworms and hookworms are very tiny and generally cannot be seen in the stool.
Tapeworms pass segments into the stool that are easy to see and are quite disgusting. Most dogs and cats get tapeworms by eating fleas, but they can also get them by eating mice, birds and rabbits.
Coccidia is an intestinal parasite that is not contagious to people. Coccidia are microscopic organisms that invade the wall of the colon and cause colitis, often with blood loss. A puppy or kitten gets coccidia by living in less than sanitary conditions, usually with a lot of other puppies or kittens.
People can get roundworms, hookworms and whipworms if they inadvertently eat stool of a dog or if they eat soil that has eggs in it. This is why children especially are at risk. These parasites can invade the skin and cause serious, potentially fatal health problems.