Science points towards mild traumatic brain injury being accumulative, leading to neurodegenerative illness:

Data from a large Danish study (100,000 patients) of whiplash associated disorders reports of an 1.3 – 2 fold increased risk of a range of diseases AMONG ALL victims to whiplash associated disorders, compared to people without such diagnosis.   (ref)

Among the associated disorders were: 

Certain infectious and parasitic diseases 1.50p<0.001
Neoplasms 1.13p<0.001
Endocrine, nutritional and metabolic diseases 1.24p<0.001
Mental and behavioural disorders 1.70p<0.001
Diseases of the nervous system 1.67p<0.001
Diseases of the eye and adnexa 1.38p<0.001
Diseases of the ear and mastoid process 1.48p<0.001
Diseases of the circulatory system 1.33p<0.001
Diseases of the respiratory system 1.46p<0.001
Diseases of the digestive system 1.46p<0.001
Diseases of the skin and subcutaneous tissue 1.33p<0.001
Diseases of the musculoskeletal system and connective tissue 1.81p<0.001
Diseases of the genito-urinary system 1.43p<0.001
Symptoms, signs and abnormal clinical and laboratory findings not elsewhere classified 1.84p<0.001
Injury, poisoning and certain other consequences of external causes 1.99p<0.001
Factors influencing health status and contact with health services 1.61p<0.001

Discussion:
First of all 90% return to work after a whiplash injury. These are included in the above statistics of 1.3-2.0 fold increase in associated disorders. Among the 5-10% chronically ill, the likelihood of an associated disorder is perhaps 10 fold larger than repotted by the study.

Second of all, lots of people remain undiagnosed after mild traumatic neck injury; thus polluting the “reference group of uninjured”.

Study find significant higher risk of associated disorders after whiplash injury compared to uninjured. Whereas they cannot identify differences before and after each neck injury. This points towards graduate buildup neck injuries, until the drop that makes people consult a medical doctor. Similar add on effect is seen in mild traumatic brain injury.

Reference