With any microbial disease, or other externally-applied shock to an organic system, Nature seems to have it that there will always be a fraction which survive. Hence the concept of LD50. And if ya give everyone typhus, a fraction will survive.
Consider the medical industry as being an externally-applied shock. Many will naturally succumb, while a fraction will be immune to it. You all's a part of that immune fraction, whilst the masses get mass-acred.