Google "toxocariasis" (the disease) and "toxocara" (the parasite that causes it)
kidshealth.org/parent/infections/parasitic/toxocariasis.html
www.medic8.com/infectious-diseases/toxocariasis.htm
Two posts from another forum (neither of them mine):
The "crap on the grass" argument is caused by the known bacterial and parasitic infections and infestations that can be carried by both dog and cat turds. You might think that by the time the turd has gone white and blown into the winds, they are no longer so dangerous, but Toxocara Roundworm eggs will stay in the soil. The "Dogs (Fouling of Land) Act 1996)" was passed in order to cut down on dog mess in public places and an 'on the spot' fine of up to £1000 is payable if you're caught.
Someone who falls into or accidentally touches dog faeces could be at risk from a disease call Toxocariasis which can lead to abdominal pain, pneumonia, asthma and even blindness. And the problem doesn't go away the longer the dog faeces is left. Toxocara eggs can take up to two weeks to hatch." - quoted from the Reigate and Banstead Borough Council site.
=====
I'm one of the very small amount of people who have actually had Toxocara. I had it 30 years back, when I was around 19 years old. I was lucky, I told my Dad that I felt 'dazzled' in my right eye, he was an optician and within the day I was being seen by his Ophthalmic doctor who tested his patients eyes. Within hours I was in Neasden Eye Hospital. I remained there for 4 weeks, not because I was ill, but because they didn't have a clue what I had..and so every day they did a myriad of tests on me. They'd isolated the infection immediately, with steriods..so I was safe.
I lost part of the vision in my right eye, but soon learnt to accomodate for it. I didn't blame dogs for that, nor cats..not even houseflies...It simply happened. It's 'life'.