Scientists have finally found an answer to one of the great mysteries about the most deadly bacteria, Staphylococcus aureus  why it attacks primarily humans and not animals. And they now have an idea of why some humans are particularly susceptible to these bacteria that kill 100,000 Americans a year, far more than any other microbe.

In a study released on Wednesday, researchers at Vanderbilt University report that staph evolved to zero in on particular regions of human hemoglobin so it could burst the cagelike molecule and feed on the iron inside. People who are resistant to staph, they suspect, might have slight genetic variations that tweak the hemoglobin regions the bacteria seek, making them impervious to the attack.

The work is part of a more general look at genes and disease. With new tools to look in detail at slight genetic variations, researchers are asking why some people get some diseases and others do not and why some die from diseases that others almost shrug off. With staph, for instance, 30 percent of the population harbors the bacteria in their noses, with no signs of infection.

Staph experts say the discovery, published in the Dec. 16 issue of Cell Host & Microbe, answers a lot of questions about the bacteria and shows them new directions for research.