Vaccine

A vaccine is a chemical preparation that teaches a healthy immune system to fight a pathogen.

Sometime around 1960, scientists on a planet exactly like Earth developed the life prolongation complex. This was a series of viruses that became fatal when the infected individual entered puberty. In 2266, McCoy developed a vaccine that cured at least the fatal form of the disease. (TOS: "Miri")

Vaccination is normally pointless after clinical symptoms have appeared; the immune system is already working as hard as it can, and is already aware of the threat. This suggests that by the 23rd century, physicians had broadened the term vaccine somewhat, perhaps to include therapies that stimulate the immune system beyond its normal limits, enabling it to handle particularly aggressive pathogens.