They are fraudulent and potentially dangerous scams.  As a physician, I used to get them in the mail years ago; always from Nigeria, and usually from a Dr. so and so.  I think they were targeting "high profile" professionals like doctors and dentists, at least initially.  I am not surprised they are now Emailing. 

Word of warning:   They are to be ignored completely and never acted upon. Delete, do not reply and forget.   Cases of abduction and murder have resulted in people who have actually ventured to Nigeria to claim their "fortune", or robbery, when they strive to access your bank account (in order to deposit "your fortune").