The correct answer for humans developed antibiotics to fight bacterial infections.
After a certain period of time, diseases and other sicknesses evolve along with humans, making the antibiotics nearly or completely ineffective. For example, if you took one tylenol when you have a cold, it may not be as effective the next day or so and would need to double the dose. Obviously it will all depend on the illness, and what stage it is in, and the strength of the medication. Also, at one point a vaccine for rabies might need an advanced formula because it might have evolved in animals. The possibilities are endless.