Cures every disease known and unknown (except a few mental disorders like Asperger's Syndrome or high-functioning autism because it isn't dangerous and not really a disease. If a person with low-functioning autism drinks the Serum, his/her condition will become high-functioning autism or Asperger's). However, it's extremely difficult to find and extract. It can only be found in the golden apples grown by indestructible trees in Purification Shrines, and such Shrines contain thousands of deathtraps.
At most, it takes about 5 minutes for the Serum of Purification to completely cure a disease. It cures by annihilating any and all pathogens and cancer cells on contact, and heals any damage done to the body. Also removes any sclerosis. Does not harm non-cancerous body cells or non-pathogenic microscopic organisms.