>>14098221The comic is nonsense, OP. That isn't how it works at all. Everyone that catches a viral disease will create an assload of new viral particles with somewhere between 0 and a few mutations each. Many of these particles will be rendered entirely harmless, while some will be rendered more/less viral or lethal or with more/less visible symptoms.
It's all a lottery, and a vaccine that actually works will only eliminate you as a potential vector for new mutations. A vaccine that doesn't work can end up like Marek's Disease in chickens, which masked symptoms but did not stop its spread, eventually promoting more virulent strains that are now a major threat to unvaccinated chickens.
Normal, common diseases tend to become less deadly over time, because a host that has a mild cough is a better spreader than a host that becomes incapacitated and dies violently. (The opposite to this would be a long-term insidious disease like HIV, which has no reason to become less lethal but all the reason to become more contagious).
