What sort of disease is a concussion?

Brain injury. A concussion is a mild traumatic brain injury. Mild in that there is not typically any visible (with an MRI or ct) evidence of structural injury. The injury is at the microscopic (cell/molecular level). Seen with falls, car wrecks or in sports, they often result in headaches, memory loss, feeling "in a fog, " nausea/vomiting, dizziness & can be associated with long-term effects like depression.
A serious one. Concussion is often due to head trauma like in falls, motor vehicle accidents or sports and needs to be evaluated. I will share this question with experts in the many fields who see people with concussions.
Potentially Serious. More and more is being learned about concussions. Obvious symptoms may include fatigue, nausea, blurred vision, pain upon light exposure. And headaches. Subtle symptoms include " muddy thinking", slow reflexive response, poor school work etc. Final resolution may take months. There are validated tests and specialists for proper evaluation and they should be used.
SHAKEN NOT STIRRED! To concus means 'to shake violently'; which is what happens to the brain when the head or trunk is violently impacted. This violent event causes brain injury at the cellular level, which is why symptoms and recovery are so challenging to gauge. The key to recovery is close follow up with a provider knowledgeable in the concussion who'll monitor symptoms & neurocognitive test results.