Italian researchers have isolated the virus implicated in the type of diabetes that mainly strikes children.
Working with Novartis Vaccines, scientists from the universities of Siena and Pisa have found the virus, Coxsackie B4, in the pancreases of people with Type 1 diabetes.
The medical world has suspected that the bug was involved in the disease for 40 years, but this is the first time it has been verified.
"Up till now researchers had only been able to posit the involvement of this pathogen in indirect ways," said Stefano Censini of Novartis Vaccines.
"Now, for the first time, the viral infection has been detected in the body, unequivocally".
Francesco Dotta of Siena University said there was a "direct link" between the virus and the breakdown of the body's auto-immune system that typically occurs in type 1 diabetes.
The pancreatic cells that tested positive for Coxsackie B4 showed key indicators of the condition including an inability to produce insulin, he said.
Commenting on the study, published in the Proceedings of the National Academy of Sciences, top Italian endocrinologist Paolo Pozzilli was more cautious.
"One cannot say that the virus itself caused the damage," he said.
Type 1 diabetes remains a disease that can be caused by various factors," he said.
Type 1 diabetes is usually diagnosed in children and young adults and was previously known as juvenile diabetes.
In type 1 diabetes, the body does not produce insulin.
Insulin is a hormone that is needed to convert sugar (glucose), starches and other food into energy needed for daily life.
Type 1 diabetics have to have frequent insulin shots - usually once a day.
The other main form of the disease, Type 2, is marked not by an absence of insulin but an inability to use it.
It is influenced, but not caused, by a variety of factors including genetic predisposition, bad diet, lack of exercise and obesity.
Like Type 1, it is chronic and deadly unless treated.
The root cause of both types remains a mystery.
