Influenza is one of the most hypervariable viruses that we know. In any one infected patient, the influenza virus is mutating literally thousands upon thousands of times.
Most of these mutations destroy the virus, but the ones that don't, survive. If just one mutation makes the virus more pathogenic, it will be rapidly spread to other organisms (er, people), and cause more illness.
So how do viral resistances develop? Say, in a given patient, 5 out of the 1,000,000 viruses in his lungs mutate to be resistant to tamiflu. The doctors then administer tamiflu, and only the 5 mutated viruses survive. Those mutated viruses then multiply, until you have 1,000,000 tamiflu-resistant viruses.
One opportune sneeze, and those tamiflu-resistant viruses spread to others.
Now do you see why have just one million people who refuse to get vaccinated will provide a MASSIVE breeding ground for the virus to mutate? If just one person who refuses the vaccine, mutates a version of H1N1 that circumvents our vaccine, we're fucked.
The vaccine-resistant H1N1 will then go on to re-infect everyone, thus starting the deadly 1918-style waves.
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