Toxicarasis can cause blindness, stomach upsets, sore throats and asthma. It is caused by a roundworm called toxocara, which is commonly found in dogs, and almost all puppies.
Eggs from the roundworm are found in dog waste, where they can pose a health risk if ingested, particularly to small children.
The eggs remain active in the soil for years, long after any dog waste has weathered away, so the risk isn't always obvious; that is why it is important to always wash your hands after using open spaces. You should always wash your hands after having contact with a dog, as dogs also carry worm eggs in their coat.