OPMV is ophidian paramyxovirus.
It is a nasty virus. Oftentimes, there are no outward symptoms and you wake up to a dead snake. In larger collections in can spread rapidly as one of it's modes of transmission is through the air.
Othertimes, the animal goes off feed and then drops dead. Or, in some cases, there are signs of respiratory distress, and abnormal activity - lack of tongue flicking, listlessness or the snake displaying "death-throes".
Death throes are what we often see when a snake is terminally ill and it thrashes or convulses about. My vet says these are known as agonal movements. In some diseases, these movements are mistaken for neurological damage or disease. The animal will corkscrew about, hold it's head at weird angles and fail to right itself - sound familiar?
I'm far from an expert on OPMV but it can be easily researched.