Tracing the origins of HIV has been a Herculean task and often not a glamorous one. Most recently, it involved the serious scrutiny of 599 samples of ape feces.
But after two decades of work, scientists have slowly pieced together a biography of the human immunodeficiency virus — where it was born, what its ancestors were and possibly how it grew up to be the mass murderer of the modern era.
Not since the Middle Ages has one virus cut such a long, wide swath through humanity. More than 25 million people have died since HIV was first recognized in 1981. More than 40 million have been infected. Some experts predict that by 2020, AIDS could prove to be the most destructive pandemic in history. Yet in all likelihood, HIV was not the first virus of its kind to infect humans, and scientists suspect it will not be the last.
The closest ancestor of the AIDS virus lived in West Africa's chimpanzees for thousands of years, and chimps have long been on the menu of human hunters in that corner of the world.
Trevor Wilson, a scientist with AnorMED, a company that is working on a drug to block HIV, examines crystal formation on chemicals this week at a lab in Langley, B.C. Rafal Gerszak/Globe and Mail
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Interactives
"Our guess would be that these viruses have been jumping from chimps into humans on countless occasions in the past," said Paul Sharp of the University of Nottingham, a leading expert on HIV's evolution. "But most of them don't make it to be an epidemic. In fact, until recently, none of them had made it out of rural areas or infected enough people to be noticed."
HIV is believed to have been killing people in Africa for roughly 50 years before the world knew the virus existed. Yet scientists have since discovered the pandemic virus was one of three types of HIV that jumped to humans around the same time. But due to a fateful confluence of events and genetic accidents, only the one — HIV-1, group M — gave rise to the global scourge that continues to stump medical science.
The king of freeloaders
HIV belongs to a group of pathogens known as retroviruses. They were once considered rare, medical curiosities and only late in the 20th century were they discovered to actually infect humans.
They have no ability to replicate outside of a host cell and they carry their genetic material not in the double-stranded code of DNA, but the single strand of RNA that makes proteins.
HIV is a retrovirus with just nine genes tucked inside a round protein envelope. Magnified, it looks a little like the wheel of a ship, circular with spikes radiating from its surface. The virus makes its way through the world on rivers of bodily fluids, striking humans at their most intimate points of contact — sexual intercourse, childbirth, breastfeeding. Tainted blood and intravenous drug use have also been sources of transmission, and HIV has developed uncanny methods to spread itself. Outside of a human host, it's powerless.
"It's a bit wimpy that way," said Richard Harrigan, director of the research laboratories at the B.C. Centre for Excellence in HIV/AIDS. A few hours outside of a human body and the virus is no longer infectious, he said; nor can it be easily grown in a lab dish.
But inside a T cell, the story is dramatically different. T cells are lymphocytes, or white blood cells, that orchestrate the body's ability to fight infection. The HIV attacks them, specifically a type known as CD4, like a guided missile.
The virus breaks in with its own set of keys and slips into the host cell like an unwanted guest. Then it settles down to become the king of freeloaders, injecting its own genetic material into the DNA of its host and making an HIV factory of the very cell that was designed to kill it.
New viruses eventually burst out of the host cell to wreak fresh rounds of destruction. "Integrating itself directly into the DNA of the host means you can't get to it easily to cause its destruction without the risk of damaging the host's cell," Dr. Harrigan said. "It also makes lots and lots of copies of itself every single day."
How many copies?
"Oh," he said, "about 10 billion."
It started with chimps
Chimpanzees had long been the suspected source of the AIDS pandemic. HIV bears a close genetic resemblance to a chimp infection known as simian immune deficiency virus, or SIVcpz.
