First, what is herd immunity?
Germs succeed when they infect lots of people. People succeed when they don't get infected. Germs spread from one person to another. Vaccination stops germs from infecting individuals, benefiting each vaccinated individual. But if enough people get vaccinated, the benefit extends beyond the individual. That's because vaccinated people don't get infected and thus can't spread germs to other people. If an individual is completely surrounded by vaccinated people who don't spread germs, then that individual probably won't get infected either--even without the vaccination.
Herd immunity kicks in when a certain percentage of the population is vaccinated. Below that percentage, enough unvaccinated people come in contact with each other for germs to spread. Above that percentage, germs mostly encounter vaccinated people so they can't spread. The germs are effectively stopped from spreading.
That magic percentage (the "herd immunity threshold") depends on how many so-called "secondary infections" an infected person can cause. Mumps usually spreads from an infected person to between 4 and 7 others, and the herd immunity threshold is between 75% and 86%. Measles usually spreads to somewhere between 12 and 17 others, so the herd immunity threshold also goes up to between 83% and 94%.
How does that tie in with self defense -- fighting back against bad guys?
Well, Lawdog points out that even bad guys operate on a risk/reward basis:
All creatures on this little green dirtball operate on a Reward/Risk system: is the potential reward of this action worth the risk of this action? If so, you do the thing -- if not, you do something else.But most of them (the "critters") only understand immediate consequences, not delayed consequences:
The possibility of getting caught by the police next week, or next month, has very little influence on the average critters Risk/Reward calculations.Just like germs, the critters succeed when they encounter a victim who doesn't fight back. And just like germs, the victims succeed when they do fight back.
Getting caught, injured or killed during the crime? Yes. Getting caught, injured or killed after committing the crime? Not so much.
[S]uppose Joe Critter is in a place where self-defence is expected and encouraged. He figures the reward of wallet money is worth the risk of Rehabilitation Through Reincarnation, or Bodily Injury and attempts a mugging. The victim defends him or her self, and let us postulate that Joe scrambles away with powder burns and a bloody furrow along the ribs.But just as with vaccination, the benefits extend beyond the individual victim. Just as a germ can't tell whether a potential infectee is vaccinated or not, it's hard for a critter to predict whether a potential victim will fight back. If enough potential victims tend to fight back, the potential critters won't bother even trying. So even the potential victims who wouldn't actually fight back don't get attacked.
[....]
This time -- self-defence being expected and encouraged by this society -- Joe crawls to the nearest trauma centre with a .38-calibre lead slug in his belly. Pretty sodding quickly, the Risk (Death or Serious Bodily Injury) is going to outweigh the Reward (wallet funds), and Joe is going to turn to an activity with a lower Risk variable.
What is the "herd immunity threshold" for fighting back? This time the math works backwards in favor of the good guys. Instead of infectees spreading germs, the fighters spread protection. Would you attack someone if you estimated a 50% chance of instant death? How about 20%? 10%? 5%? Critters do the same math. Their threshold may be higher, but most of them still don't want to die.
That's why it's so important to fight back. Not only do you stop your own victimization, you protect other people from attacks who wouldn't otherwise fight back.
You can even do it "for the children"!










