Aids: Evolution Day by Day

Bacteria are not the only parasites that evolution has turned into a worldwide menace. Over the past few decades, the human immunodeficiency virus–the cause of acquired immunodeficiency syndrome, or AIDS–has evolved from obscurity into a global epidemic.

Viruses such as HIV are peculiar as parasites go. They are not alive in the sense that bacteria or humans are. They do not have a metabolism that allows them to extract energy from food and release wastes. They are simply small collections of DNA or RNA encased in a protein shell. When they invade cells, their genetic material commandeers the protein‑making factories of their host. Their host cells make new copies of the virus, which burst out of the cell and search for new homes.

In their own way, viruses are just as ruthless as bacteria in their coevolution with their hosts. They do not have the cellular machinery that bacteria have to swap genes, but they can more than make up for this shortcoming by evolving on overdrive. The genome of HIV is only 9,000 base pairs, compared with 3 billion base pairs found in human DNA. But when a single virus infects a new human host and invades a white blood cell, it starts multiplying wildly. Within 24 hours, it has turned into a horde of viruses billions strong.

Almost as soon as the virus starts multiplying, our immune system starts recognizing infected white blood cells and destroying them, wiping out the viruses in the process. But despite the immune system’s ability to kill HIV by the billions every day, HIV can survive these attacks for years. The secret to its longevity is its ability to evolve. The enzymes that HIV uses to make new copies of its genes are very sloppy, making one or two mistakes on average every time they duplicate the virus’s genome. Among the many mutants that spring up, a few strains will turn out to be hard for the immune system to recognize. Because HIV replicates so quickly, these resistant viruses quickly become the dominant strains in a person’s body. It takes time for our immune system to shift its attack toward the new strain, and once it does, the viruses evolve even newer forms that escape the immune system yet again.

Virus and host remain balanced in this equilibrium for years, poised between population explosions and implosions. Without an HIV test, infected people have no way of knowing that a coevolutionary struggle is raging under their own skin. Only when HIV has destroyed the immune system, allowing other parasites to invade and causing full‑blown AIDS, does the virus make itself known.

Drugs now exist that can interfere with the enzymes HIV uses to replicate itself, and they can slow the progress of AIDS. Yet even though anti‑HIV drugs have been available only for a few years, the virus’s mutational overdrive is already threatening to make them useless. Just as the virus can evolve around the latest wave of attacks from the immune system, it can mutate into forms that cannot be harmed by drugs. Even one or two mutations are enough to overcome these drugs. In a matter of weeks, a patient’s load of HIV can bounce back up to the level it was at before the treatment began.

Switching to a different drug makes it possible to kill off most of these resistant viruses, but among the survivors a new mutant may emerge that can resist the new treatment. Doctors therefore favor giving their patients cocktails of several drugs at once. A virus may be able to evolve resistance to a single drug with one or two mutations, but it is much less likely to escape several other drugs at the same time. Yet even under attack from drug cocktails, multiple‑drug‑resistant HIV is emerging.