Replies

Six hundred and fifty years ago, the Black Death was stalking Europe. It arrived on ships from Asia, carried by fleas that had infected rats on board the ships. Before it burned itself out, the epidemic had killed about a third of the European population.

Today, another plague - AIDS - has ravaged the world. Although it seems very different from the Black Death, there is one eerie similarity. Both the Black Death bacteria, Yersinia pestis, and HIV, the virus that causes AIDS, home in on macrophages, which are scavenger white blood cells of the immune system.

Now, in a provocative report, scientists at the National Cancer Institute in Frederick, Md., say they have found that a genetic mutation that protects against the AIDS virus, by preventing the virus from entering macrophages, emerged in Europe around the time of the Black Death.

And, they have found, this AIDS resistance gene is astonishingly common in people whose ancestors lived in areas of Europe that were ravaged by the Black Death.

The HIV resistance gene destroys a protein, called CCR5, that pokes out of the surface of macrophages, the large white blood cells that can engulf and kill viruses and bacteria.

Scientists have discovered that when HIV infects a person, the virus goes straight to the white blood cells and in particular the macrophages, latches onto CCR5 and another protein, CD4, to hoist itself inside.

It lives there for about a decade, throwing off billions of genetic variants. Eventually it makes a variant that can get into another type of white blood cell, the T cells. Then the infected person's immune system starts to decline, and the terrible symptoms of AIDS appear.

People who inherit two copies of the HIV resistance gene can only be infected with HIV if they happen to come in contact with a virus from someone in the late stages of infection, when the virus can go straight for the T cells.

10 percent of Caucasians have a copy of the gene, which slows the progress of HIV infections by several years, and one percent have two copies, which provides nearly complete immunity to HIV.

The HIV resistance gene is most common among British and other northern European people, and declines in frequency further south.

Thus, it is present in almost 14 percent of Swedes but appears in only about 5 percent of Italians and is absent in Saudi Arabia. It is absent in Africans, American Indians and Asians. The gene emerged in the Caucasian population long after Caucasians split from Asians, which was about 50,000 to 100,000 years ago.

And so, although the bubonic plague began in Asia, the HIV resistance gene is not there.