[excerpt] Not so with prion diseases. Researchers haven't found a single germ or bug that causes them, and the immune system appears to snooze right through the infection. "Prion diseases seem to represent a whole new class of infectious agent," said Byron Caughey, an investigator who studies prions at the National Institutes of Health.
Prions (pronounced PREE-ons) are one of countless types of proteins found in normal cells, and are at the center of the mystery. Scientists, who identify proteins by their shapes and molecular structures, know the job descriptions of many of them: Some act like bricks in the structure of a cell, while others help cells carry out chemical reactions or communicate with other cells. Prions appear to be important since there are so many of them in our brain cells, but, like the boss' son-in-law, their role in the whole organization is unclear.
In fact, one British researcher found that genetically modified mice without prions look and act exactly the same as other mice. "You can't talk to a mouse, and maybe it's lacking in some function, but as far as (the researcher) could tell, the mouse was perfectly normal," said David Eisenberg, who studies prions at the University of California at Los Angeles.
"But it could be that the tests one could perform on the mouse aren't sensitive enough to reveal the differences," he said.
There was an important difference, though: Without prions to call their own, the mice couldn't come down with prion disease.
People, of course, aren't so lucky. At least two kinds of prion disease infect humans who eat the wrong thing -- variant Creutzfeldt-Jakob disease, apparently caused by eating meat from cattle infected with mad-cow disease, and kuru, which infected members of a New Guinea tribe of cannibals. It's not clear how infected prions make their way from food to the brain of a victim.
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