While some soldiers coming home from Afghanistan may look fine, soon it's apparent they're not. They harbor a hidden injury, called traumatic brain injury, or TBI.

Their memories are hit and miss. They get headaches, can't concentrate, are irritable, and often depressed. These are just a few symptoms of TBI, which is now known as the signature injury of the Afghanistan and Iraq wars.

What's even more troubling: a new study shows these soldiers face over twice the risk of early onset dementia and other diseases of the brain.

The study analyzed the records of 280,000 veterans age 55 or older. Of the vets who reported a TBI at the start of the study, 15 percent developed dementia seven years later. Compare that to just 7 percent of non-TBI vets who developed dementia.

The implications are enormous. According to the Pentagon, over 200,000 soldiers have suffered a brain injury over the past ten years. Other sources put that number even higher.

Traumatic brain injury happens when the head suddenly and violently hits an object, or when something pierces the brain. Soldiers suffer TBIs from roadside bombs and explosions, which account for 59 percent of the injuries in these attacks.

More studies are showing that just one severe head injury increases the risk for Alzheimer's. For example, within hours of a severe TBI from a car crash or fall, a protein called amyloid-beta starts accumulating in the brain, creating a plaque characteristic of the disease.

Repeated, mild TBIs lead to the build-up of a different protein called tau. This abnormal protein accumulates in nerve cells, clogging them up and eventually killing them. Over time, they destroy the brain's ability to function normally.

Soldiers with TBI may be at risk for developing both types of dementia. The question now is whether early treatment after a TBI can help prevent the onset of dementia in the future.