About 3,400 years ago, a mysterious plague is believed to have spread across Europe, killing vast numbers of people.

No written records of the unknown disease survive today.

But scientists at the University of Ottawa Heart Institute have helped to uncover another piece of evidence in the genes of modern Caucasians.

A small cluster of genes protected part of early Europe's population against a disease that must have been horrific, perhaps on the scale of the Black Death.

But there was a cost: those genes, still carried by many today, raise the risks of heart disease, diabetes and hypertension.

''Thirty-four hundred years ago I think we can imagine that something cataclysmic happened to humans, probably an infection or a plague that wiped out 90 per cent of people,'' said Alex Stewart, a geneticist and microbiologist at the Heart Institute.

''It could have been almost any viral infection. Could have been reminiscent of Ebola.

''The people that were able to survive and have children, and have the children survive - they had to have something really resistant to the infection. ''

Now, 3,400 years later, the protective genes are still there and they're hard on the heart.

''All of a sudden, what was beneficial then is detrimental now,'' said Stewart.

About half the Caucasian population has at least one copy of this gene cluster and some have two, he said.

Yet people from Asia and Africa have none, leading researchers to believe they weren't exposed to whatever disease hit Europe.

The time around 1400 BC was just before King Tutankhamen ruled Egypt.

Egyptians would have left written records of an epidemic, but Europeans of that time did not.

Stewart joined researchers from 43 other hospitals and universities for the study, published Monday in the journal Nature Genetics.

The study was looking for genes that cause problems involving red and white blood cells, platelets, and hemoglobin levels.