According to the EPA:

Why is it important to manage pet and wildlife waste near the sources of your Drinking water?

Probably the greatest health concern associated with animal wastes is pathogens. Many pathogens found in animal waste can infect humans if ingested. Organisms such as Cryptosporidium, Giardia lamblia, and Salmonella can induce symptoms ranging from skin sores to chest pain. E. coli, which causes diarrhea and abdominal gas, has been the source of disease outbreaks in several States. Particularly virulent strains of E. coli can cause serious illness and fatalities. Cryptosporidium is of particular concern because it is highly resistant to disinfection with chlorine. This protozoan causes gastrointestinal illness lasting two to ten days in healthy individuals but can be fatal in people with weakened immune systems. Dog and cat droppings often contain roundworms and other parasitic nematodes. Infection by just a few roundworms usually causes no problems, but more severe infections may cause fevers, bronchitis, asthma, or vision problems. Cat feces may contain toxoplasmosis, a parasite that infects humans and other animals. Cats are the only animals known to excrete toxoplasmosis oocysts, which are resistant to most disinfectants. Toxoplasmosis is a serious health concern for pregnant women and immuno-compromised individuals."

According to the FDA:

"Worms, such as roundworms and hookworms, can infect dogs, cats, and some other animals. Worms can also infect people if they ingest the organisms or, in the case of hookworms--which can penetrate the skin--if they walk barefoot on infected soil."

Worms live in the intestines of animals and are expelled in the stool. If left untreated in pets, homes and yards can become contaminated from worm eggs that are passed in animal feces and hatch in the soil. If your animal has worms, get it treated and clean up after it promptly, advises Linda Wilmot, D.V.M., a veterinary medical officer in the FDA's Center for Veterinary Medicine (CVM). 'Don't give the eggs that are passed in the feces time to hatch."

According to the CDC:

"Hookworm eggs can develop into infective stage larvae in the soil in as little as 5 days, and ascarid eggs within 2 weeks, depending on temperature and humidity.   To illustrate the extent of environmental contamination that can occur as the result of one infected puppy, a single female ascarid can produce more than 100,000 eggs/day, resulting in millions of potentially infective ascarid eggs per day spread throughout the area the puppy is allowed to roam. Once the eggs become infective, they can remain infective in the environment for years."

"Just one gram of feces contains 23 million fecal coliform bacteria"