It has been documented just how having a dog can help autistic children - as this clip shows:
But what I find really exciting is the news that some dogs can detect cancer in people just by smell! I can't face paraphrasing and typing all this so have copied it from http://www.dogsdetectcancer.org:
"Dogs can smell in parts per trillion. An example of this is: one cc (less than a drop) of blood, diluted into 20 Olympic sized swimming pools. . . . We have trained dogs to sniff gun powder, narcotics, missing persons, and now, finally, diseases. The interesting part about this is that cancer absolutely has a smell. Most oncologists will tell you that humans can actually smell cancer in latter stages through the patients breath. If we can smell it at stage 3-4, then of course a dog would be able to detect the scent much earlier, in stage 0, 1 or 2. There are many published studies that prove dogs can detect cancer through breath samples, and scientists and doctors are trying to come up with a breathalyzer test that works as good as the dogs nose. So far, the only ones that can smell cancer in early stages, are the dogs."
I find that absolutely astonishing.