Helmets protect your head—but they can't fully protect your brain. This helps to explain why football players continue to incur brain trauma that may lead to debilitating brain disease.

Recently, a team of researchers presented more evidence of the devastating progression of a brain disease caused by repeated brain trauma. On December 2, researchers from Boston University, the U.S. Department of Veterans Affairs, and other institutions published findings in the journal Brain that document the changes caused by chronic traumatic encephalopathy (CTE).

Over the course of this neurodegenerative disease, those afflicted experience symptoms ranging from headaches and difficulty concentrating in Stage 1, to the dementia and aggression of Stage 4. Repeated mild traumatic brain injuries such as concussions cause CTE, making the disease of special concern for athletes and military personnel.

The researchers posthumously analyzed samples from the brains of 85 subjects, including athletes and veterans, with a history of traumatic brain injuries. These samples revealed the disease's progressive course through the brain, including the presence of tangled tau proteins, a marker of cognitive decline also associated with Alzheimer's disease. In addition to documenting the disease's pathology, the study bolsters the argument that head injuries sustained by athletes may contribute to this disease. Of the 85 subjects, the scientists identified 68 cases of CTE and 64 were in individuals who had played contact sports such as football or hockey.

But football and hockey players wear helmets—shouldn’t they protect players from the trauma of a head-on collision? As the video below from Yale University's Ainissa Ramirez shows, helmets are designed to protect the skull—not the brain. The brain can be hurt as it smashes against the skull, causing a range of symptoms including headaches and loss of consciousness.