Alzheimer's disease, though common, remains shrouded in mystery — its cause still poorly understood. But scientists recently made an important breakthrough, discovering five new genes that shed light on why and how certain people develop the brain-deteriorating illness. Though the history of Alzheimer's treatment is littered with false hope, researchers believe this latest discovery is a step on the road to better treatment, and perhaps even prevention. "We have a lot left to do to complete the story of Alzheimer's genetics, but this is a big step," says Gerard Schellenberg, who spearheaded one of the studies. Just how big is it? Here's a brief guide:

Just how widespread is Alzheimer's?
Very. There are 35 million people with the disease worldwide — 5.4 million of them in the United States. One in eight Americans over 65 suffer from Alzheimer's.

And what exactly did these scientists discover?

By extracting DNA samples from both healthy and Alzheimer's-stricken subjects — then analyzing which genes appeared more frequently in those who carried the disease — scientists discovered five new genes that are associated with Alzheimer's. That doubles the number of known genes linked to the disease. Some of the newly-discovered genes are also associated with high levels of cholesterol, and others with an increase in brain inflammation.

So high cholesterol can lead to Alzheimer's?
Yes, people with high cholesterol are more susceptible, says Gina Kolata at The New York Times. Such risk factors, however, only increase the chance of Alzheimer's by 10 to 15%.

Does this crack the Alzheimer's case?
Not exactly. "Think of the genes as clues to the underlying causes of Alzheimer's," says Marissa Cevallos at the Los Angeles Times. Scientists still don't know exactly what causes the disease. They know that plaque-forming proteins build up in the brain in patients with Alzheimer's, though they don't yet understand why. But by throwing in cholesterol and brain inflammation as risk factors, the new genes provide clues that will hopefully lead to a clearer understanding of the disease.

Sources: Nature Genetics, New York Times Bloomberg, Los Angeles Times