In 2010, scientists found a new fish virus that was named Piscine Reovirus (PRV).
The virus is common in farmed fish and it is thought to cause HMSI in salmon. The virus is also found in wild salmon, but no one has been able to detect disease.
Researchers at the National Veterinary Institute and Natural History Museum have conducted a genetic analysis of the virus which shows that it is the same PRV type in both farmed and wild fish.
This suggests that infection occurs between the two groups, but it is not easy to research the virus in wild fish.
"It is difficult to catch sick wild fish to sample because they die and disappear or are eaten by other animals," said Eirik Biering at the National Veterinary Institute.
He believes this is one reason why studies of wild show few discoveries of dangerous viruses and bacteria.
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