WASHINGTON, D.C. (AP) - Secretary of State Hillary Rodham Clinton, who contracted a stomach virus in early December, became dehydrated, fainted, fell and hit her head, according to aides and doctors. She was diagnosed with a concussion on Dec. 13 and hasn't been seen in public since as she follows orders to rest and recover. That has meant, among other things, canceling an overseas trip to deal with Syria and North Africa issues, the release and congressional hearings about an independent panel's report on the deadly Sept. 11 attack on a diplomatic mission in Benghazi, Libya, and on Friday, standing with President Barack Obama as he nominated Sen. John Kerry to replace her as secretary of state for his second term, as expected.

President Obama, in nominating Kerry, noted Clinton's absence. "She continues to recuperate," he said. "She's in good spirits."

After talking to her Friday, he said she looks forward to returning to work, but didn't say when.

Here is a look at how a concussion affects the brain, and the recovery process:

—A concussion is the mildest form of traumatic brain injury, and usually involves losing consciousness after a fall or blow to the head. Symptoms can include headaches, irritability, sleep problems and sensitivity to light or noise, according to the federal Centers for Disease Control and Prevention.

—Most people who suffer concussions recover fully, but that can take "days to weeks to months depending on how bad it was," said Dr. Gholam Motamedi, a neurologist at Georgetown University Medical Center who was not involved in Clinton's care.

—How hard a surface Clinton struck when she fell, and whether the blow was to the front or back of her head (back is worse) can affect the severity of her injury, he said.

—Treatment usually focuses on getting enough rest and sleep to allow the brain to heal, and preventing further head injury — avoiding strenuous activities or sports, even working on computers for a while. Doctors also like to make sure that blood pressure is controlled and that there are no other problems or injuries to complicate recovery.

—Doctors often advise "kind of keeping a low profile" during recovery and boosting general health by eating well and resting, Motamedi said. "You want the brain to be in optimal condition" to heal.