Though polio affects the nervous system, in humans it is transmitted through the gastrointestinal tract. Pathologists had discovered the poliovirus in human intestines as early as 1912, suggesting entrance through the digestive tract. 
Monkey “models” falsely indicated that only brain tissue could be used to grow poliovirus. Monkey studies also led researchers to conclude that the virus entered the body through the nose and spread directly to the central nervous system. 
The animal studies conveyed both the wrong model of transmission and the wrong course of the disease for the human body. So researchers spent decades studying and infecting the nervous systems of primates with the poliovirus, but it was a dead end.
In 1954, three scientists received the Nobel Prize for Physiology and Medicine for growing the poliovirus in human non-nervous tissue culture. Albert Sabin, one of the developers of the polio vaccine, later testified before Congress that animal models of polio actually delayed the vaccine’s development for decades. Sabin said: “… polio prevention was long delayed by the erroneous conception of the nature of the human disease based on misleading experimental models of the disease in monkeys” 
The search for treatment for diabetes
Clinical data, combined with basic biochemistry techniques, led to the discovery of insulin. Insulin was not discovered through animal experiments. The standard claim by animal researchers is that Charles Best and Frederick Banting discovered the hormone using dogs. But this is not true.
Insulin is a hormone produced by the pancreas. In diabetic patients, there is an absence or deficiency of this hormone, which regulates glucose (sugar) metabolism. In 1788, an autopsy performed on a patient who had died from diabetes revealed a link between diabetes and some sort of abnormality of the pancreas. Acceptance of this link was delayed for more than a century largely because scientists failed to produce a similar diabetic state in animals by damaging the pancreas. 
In 1920, during an autopsy on a human patient who had died from a rare pancreatic disorder, a pathologist discovered the Islets of Langerhans, which is the site of insulin production in the pancreas and its link to diabetes. The discovery of insulin itself took place in 1921 when a biochemist isolated it from among others substances secreted by the pancreas. The substance was then tested on human diabetics and found to be effective. 
 Radestsky, Peter. The Invisible Invaders: The Story of the Emerging Age of Viruses, Little Brown & Co. 1991
 Albert Sabin, MD, Statement before the subcommittee on Hospitals and Health Care, Committee on Veterans Affairs, House of Representatives, April 26, 1984 serial no. 98-48.
 Bliss M., The Discovery of Insulin, 1982
 Opie, E.L. Diseases of the Pancreas, Its Cause and Nature, 1903