Santa Vacca
Louis Pasteur was trying this with chicken cholera. With one batch of chickens he used an old culture. The experiment failed: none of the chickens got sick. He tried again with a fresh, strong culture. The experiment failed again; still those chickens didn’t get sick. Even though the same fresh culture was sure-enough sickening other chickens who hadn’t previously gotten doses of old, dead bacteria.
“Holy Mackerel!” Pasteur said (or words to that effect in French), “I think I may be on to something.”
Vaccination is one of the single greatest contributions to public health ever made. Uncountable lives have been saved and untold suffering has been prevented because of this discovery. Now, vaccination is under attack from those who don't understand the science, from those who don't remember the horrors. They say there is no benefit, that the incredibly small chance of side effects is not worth the risk. That, despite all science says to the contrary, vaccines can cause autism.
If you've ever doubted the efficacy or necessity of vaccines, please read this. If you're a parent or potential parent, debating whether or not to vaccinate your children, please PLEASE read it. This is how lives are saved.