Which is less secure? A Windows PC bloated with spyware, or the human genome?

...all told, genetic parasites related to viruses account for more than 40 percent of all human DNA. Our body works hard to silence its viral stowaways by tying up those stretches of DNA in tight stacks of proteins, but sometimes they slip out... Sabunciyan has found that an unexpectedly large amount of the RNA produced in the brain—about 5 percent—comes from seemingly “junk” DNA, which includes endogenous retroviruses [ERVs].

Even the oldest legacy code in Windows ME is merely metaphorically from the time of the dinosaurs.

In the past few years, geneticists have pieced together an account of how Perron’s retrovirus entered our DNA. Sixty million years ago, a lemurlike animal—an early ancestor of humans and monkeys—contracted an infection. It may not have made the lemur ill, but the retrovirus spread into the animal’s testes (or perhaps its ovaries), and once there, it struck the jackpot: It slipped inside one of the rare germ line cells that produce sperm and eggs.

Humans display no symptoms of an ancient infection, except for...

...a rough account is emerging of how HERV-W could trigger diseases like schizophrenia, bipolar disorder, and MS. Although the body works hard to keep its ERVs under tight control, infections around the time of birth destabilize this tense standoff. Scribbled onto the marker board in Yolken’s office is a list of infections that are now known to awaken HERV-W—including herpes, toxoplasma, cytomegalovirus, and a dozen others. The HERV-W viruses that pour into the newborn’s blood and brain fluid during these infections contain proteins that may enrage the infant immune system. White blood cells vomit forth inflammatory molecules called cytokines, attracting more immune cells like riot police to a prison break. The scene turns toxic...

Whether people develop MS or schizophrenia may depend on how their immune system responds to HERV-W, [Perron] says. In MS the immune system directly attacks and kills brain cells, causing paralysis. In schizophrenia it may be that inflammation damages neurons indirectly by overstimulating them.

What is to be done?

Looking ahead, better prenatal care or vaccinations could prevent the first, early infections that put some people on a path to schizophrenia. For high-risk babies who do get sick, early treatment might prevent psychosis from developing two decades later.