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"Genealogy Today", by Betty Lou Malesky, CG
From the Green Valley News, Sunday 12 May 2013, page C2

Did Your Ancestor Survive the Plague?

Most of us in Green Valley have European ancestry. Europe was ravaged by the Black Death, also known as the bubonic plague in the 14th century. Few countries were spared, though it was most prevalent in southern France and the heavily populated Mediterranean area where it's believed over half the population died.

The plague originated in Asia and was carried into Europe in 1346. The Great Plague of London in the 1660's may have added to the numbers who carry immunity. Historically it was thought the plague was spread by rats, but due to the vast area affected many scientists now believe it was a virus carried person to person. Other geneticists feel plague resulted from a bacterium, Yersinia pestis.

The former believe descendants of plague survivors carry immunity against other viruses, among them the human immunodeficiency virus (HIV), which causes AIDS. Devastating epidemics that swept Europe during the Middle Ages seem to have had an unexpected benefit — leaving 10% of today's Europeans resistant to HIV. The big question is whether the immunity is actually derived from the plague.

A study published in 2003 suggests small pox as the source of the mutation, while other researchers believe plague is the source. Neil Ferguson, an infectious disease expert at London's Imperial College is one of those who favor small pox as the mutation's source.

Christopher Duncan, however, of England's University of Liverpool feels "outbreaks are easier to explain if one assumes plague was passed directly from person to person as a virus." While the 'bubonic plague' was caused by bacteria carried by rats and their fleas, "Rats are absolutely in the clear for Europe," he argues. He admits, however, that he cannot prove his theory.

Genetic mutations happen by chance. The gene in this case is named CCR5-delta 32. It is found in the gene CCR5 as a macrophage— a receptor on the surface of the immune cells. The Aids virus uses CCR5 to attach itself to cells. People with this mutation have a lesser chance of dying of Aids, increasing the numbers who have immunity with each generation.

Those who lack the receptor because they inherited the mutant gene from both parents will be immune to HIV. Those with one mutation and one normal gene have fewer CCR5 receptors. They may get Aids but will have a less potent version of the disease. Currently, it's not known whether Yersinia pestis uses CCR5 to cause an immune response.

Stanford University microbiologists plan to take cells from people with normal CCR5 genes and from those with one of two copies of the gene, infect them with the bacterium, Yersinia pestis, and compare the number of cells that become infected. Since changes occur at a relatively constant rate over time, they hope to be able to determine the age of the ancestral mutation.

Small pox infected Europe in the 1600s and may not have been around long enough to have such a big genetic effect as this mutation. Generally, the most harmful mutations pass quickly because people infected die before they can have children and pass them on.

The real danger in Yersinia pestis today is its potential use in biological warfare as a way to conquer an enemy's population without atomic weapons.

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