that's inaccurate.
A more correct statement would be...
One of the cruelest known human practices is to inject a small number of weakened bacteria into the human bloodstream to be brutally slaughtered by overwhelming numbers of leukocytes on a yearly basic, merely to boost the individual human immune system against the bacteria in it's full strength.
There is some human beings that do not condone this practice and will allow themselves to fall prey to the bacteria and suffer the consequences for that actions. These humans seem to believe it's better for them and their kin to die or suffer other forms of harm from the effects of lower lifeforms.