Mercy

Mercy was a nurse in the 1800s. She is literally made of mercury, which was popular during that time in the medical field. 

She worked as a nurse for a little over 200 years until she was fired after part of her dimethylmercury saliva (or blood/another chemical fusion, idk yet) dripped on a few to many patients over the time she worked and caused casualties. She was fired after they figured out mercury was toxic and connected the deaths back to her.