His medical history, weight, and age all added up against Ariel Sharon over the past three weeks as the Israeli prime minister tried to recover from a mild stroke suffered in December. Sadly, one stroke raises the risk of another one, as does advanced age (Sharon is 77) and obesity (he is seriously overweight). Wednesday night, heightened risk contributed to Sharon's second, extremely severe, stroke, caused by a burst blood vessel in his brain, and brought him to Jerusalem's Hadassah Ein Kerem hospital, where today he is fighting for life.

Doctors at the hospital described the stroke as "massive" and "significant," medical words that mean serious damage to a vital organ. Two operations, totaling over nine hours of surgery, to stop the bleeding indicate the hemorrhage was large. In a stroke, the pressure caused by the gush of blood kills off brain cells, leading to permanent disability and sometimes death.

"Generally, the larger the size, the worse the prognosis," says Jose Merino, a neurologist at the National Institute of Neurological Disorders and Stroke in Bethesda, Md. He adds that the location of the stroke is also very important. A stroke on the left side of the brain impairs the ability to speak and to understand. On the right side, it damages the cognitive abilities, like problem solving. A stroke in the brain stem can be very damaging, ruining the brain's control over heart rate and breathing. Sharon's doctors have not released details of stroke size and location. They have said his pupils are responsive, indicating brain activity, and that they plan to keep him deeply sedated for at least 48 hours to allow his condition to stabilize.

It's possible that drugs Sharon was given after his first stroke slightly increased the likelihood of the second one. That first stroke was caused by a blood clot, known as an ischemic stroke. Standard treatment for that type of stroke is an anticoagulant, also known as a "blood thinner," to keep other clots from forming. But such drugs by their nature increase bleeding if a blood vessel does burst.

"But that risk is very small, only 2 or 3 percent," says Merino. "And the risk of not giving the drugs and getting a second ischemic stroke is much higher. In some patients, it can be as high as 70 percent." Another neurologist who specializes in stroke, Stanley Tuhrim of Mount Sinai Medical Center in New York City, agrees. "In terms of risk versus benefit, it makes sense to use these drugs," he says.

Tuhrim does worry that Sharon's experience may scare other stroke patients away from taking anticoagulants.

"I've already gotten a few calls about that. People should not be scared of the medicine, which can help them a lot. The risk of another ischemic stroke is much higher [without the anticoagulants] than any danger."

To find out more: Go to the U.S. News guide on stroke to get in-depth information about symptoms, tests, treatment, and prevention.