Poliovirus is usually spread from person to person through infected fecal matter entering the mouth via contaminated drinking water. It may also be spread by food containing human feces and less commonly from infected saliva. Those who are infected may spread the disease for up to six weeks even if no symptoms are present. 

The disease may be diagnosed by finding the virus in the feces or detecting antibodies against it in the blood.  The disease only occurs naturally in humans.
For more information, the Wikipedia entry is a good start.