General Paralysis of the Insane
The history of syphilis is very interesting. It started as a mild skin disease in the tropics but changed to something much more lethal when it was brought to more temperate latitudes. From a port in the Mediterranean in the middle ages it spread throughout Europe, at first too aggressive for its own good - killing its host before it could effectively spread the contagion - before mutating once again to become a more effective microbe that could survive and spread in the population.
By the nineteenth century syphilis was widespread - crowding in the expanding industrial centres proving to be ideal conditions for its propagation. Once someone was infected by syphilis it was, initially, fairly obvious with primary symptoms (skin eruptions) at ‘the point of contact’ (to put it delicately). This was the primary stage and it was generally known that this could be cured using a compound containing mercury (calomel). However if the infection was not cured then it would spread throughout the body producing secondary symptoms (a rash perhaps) but after this it might lie dormant for years with no further sign and the patient would probably consider his or herself cured. However after some time (tens of years) the infected patient might suddenly start behaving oddly, exhibiting wildly extrovert and ridiculously optimistic behaviour before losing control of both limbs and mouth. They could also have fits. The symptoms were well known (though the link to syphilis hadn't yet been made), followed a set pattern and were known as 'general paralysis of the insane'. This was because the spirochete that causes syphilis had started to attack the brain - and the eventual result was complete paralysis and death.