As any science guy recognizes, evolution is a fact of life. If an environment is inhospitable to a population, that population dwindles and another prevails.

Flu shots are designed every year to prevent the premier emerging strains that are likely to become dangerous, either through virulence in the body, or through fecundity causing widest spread. So a significant portion of the human population receives this vaccine, making it unlikely for that set of strains to proliferate. The evolutionary environment, in this case, is the human population.

What’s left to proliferate? The strains that the vaccine did not cover. The not-prevented strains are therefore the ones that get the news coverage, and give ammunition to the anti-science antivax crowd. The less dangerous varieties are still diseases. The vaccine mainly suppresses the ones judged to be a bigger hazard.

So, one might well ask, why get a flu shot? Why bother?

Because the flu shot prevents the most dangerous strains, and also often reduces the effect of the ones not covered. Each cycle (call it a year) the flu evolves new attack and defense systems. Usually, the current strains share these in common. So vaccinating against one of the current strains offers at least token resistance to the others.

Think of the vaccine as similar to wearing a flak jacket. This will prevent occasional small arms fire (the occasional passing virus) from hurting you. But why (antivaxxers will ask) can unvaccinated people who are infected and sit next to vaccinated ones be a danger to a vaccinated person? Aren’t they “protected?” This is a case of heavy fire from small arms eating up the flak jacket. It is not that the vaccine doesn’t work; it is simply not able to withstand a full assault. This is why un-vaccinated people are dangerous even to the vaccinated.

Measles is a good example: This is a highly contagious virus that lingers in the air for hours, and depresses the immune system for months after the spots clear; both highly contagious and quite dangerous. A vaccinated person can walk though a room with measles in the air without worry. Unvaccinated people are likely to be sick for months after just crossing that room. But even a vaccinated person might get infected if they sit next to (or in the same airplane with) a measly person for an hour. Might. Everyone who was not vaccinated is likely to come down with measles. Then another, more dangerous ailment while their immune system is compromised. This second phase is how measles usually kills.

So the tl;dr is: Get vaccinated as soon as it is available!