Viruses play hide and seek.

• Position: Infection

Viruses achieve their definition of success when they can thrive without killing their host. Now, biologist Pamela Bjorkman of the California Institute of Technology, Pasadena, has uncovered how one such virus, prevalent in humans, evolved over time to hide from the immune system.

The human immune system and the viruses hosted by our bodies are in a continual dance for survival--viruses seek new ways to evade detection while our immune system devises new methods to hunt them down. Human Cytomegalovirus (HCMV), indicates Bjorkman, "is the definition of a successful virus--it thrives but it doesn't affect the host."

HCMV is carded by eight in 10 people. Although it generally harms only those who are immunocompromised, it has been linked with brain tumors like the one which afflicted the late Sen. Ted Kennedy (D.-Mass.). Understanding how HCMV survives may help in the development of a vaccine, as well as in the fight against other viruses with similar evasive tactics. Bjorkman describes the underpinnings of a viral cloaking device, partly made of stolen goods from healthy cells, that helps HCMV to move undetected through the body.

Bjorkman long has been dedicated to understanding class 1 major histocompatibility complex (MHC) proteins and the immune response, most recently related to AIDS research. MHC proteins carry peptides, small pieces that are chopped up from the cell's internal proteins, to the cell's surface. If a cell has been infected, MHC presents viral peptides to signal T cells to kill it. So, some viruses evolved to...