Those older than 70 years that develop non-melanoma skin cancer have a dramatically decreased risk of Alzheimer's disease in a study of 1,102 patients.
Neurology May 15, 2013
Conversely, it is known that patients with Parkinson's disease have a higher-than-expected frequency of melanoma and patients with melanoma are more likely to develop PD.
Int J Cancer. 2011 May 15; 128(10):2251-60