Bodies of the dead seals were washed up on beaches all around the North Sea coast after the epidemic of phocine distemper virus (PDV) struck in 1988.
The disease destroys the seal's immune system.
And now scientists think it's about to hit again, and this time it could be even worse for the precious seal population.
Spreading
Since May, about 1,000 dead seals have been found in Anholt off the coast of Denmark - where the last outbreak was - Sweden and the Netherlands.
Most of them seem to have died because of problems with breathing and their nervous systems.
But tests have found they were infected with PDV.
Scientists fear the disease may spread very quickly again because of the great distances seals swim to search for food.
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