immunology

It seemed like nothing at first. The red patch that appeared on Roy Brillon's thigh could have been a spider bite. But as the weeks passed, it grew and grew. By December 2004, the innocuous-looking bump had become an open wound the size of the palm of his hand. Brillon's doctor, Randy Wolcott, prescribed just about every antibiotic he could think of to cure the infection, but the lesion just got worse. "It was really bad," says Brillon, a 62-year-old retired housepainter from Lubbock, Texas. "I had to give up work because I couldn't climb ladders anymore."

Brillon felt like he was being eaten away from the inside out. And in a very real sense, he was.

It has long been known that contracting HIV through oral sex is rare. Klara Hasselrot of Stockholm's Karolinska Institutet recently wrapped up a study--detailed in a forthcoming paper in the international AIDS journal AIDS--that might shed some light on why this is. It provides the first-ever evidence that humans can develop resistance to HIV in their saliva.

Welcome to the inaugural post of The Sex Files. Almost every publication worth its druthers has a sex column these days, full of Carrie Bradshawish musings about life and love, men and women, this and that. Here's our take on the genre. Instead of faux-sociology, we'll give you a broad view of new research and ideas in the sexiest of the hard sciences: reproductive biology, evolutionary anthropology, and genetics. This is sex from the inside out. Keep track of the column at popsci.com/sexfiles, where you can also sign up for an rss feed.

Disassortative mating alert! A group of European scientists led by Oxford biostatistician Raphaelle Chaix has provided some of the most compelling evidence yet that we humans pick our partners based on how different their immune systems—or officially, their Major Histocompatibility Complexes—are from our own.