You know why? Because a flu shot trains your immune system to be lazy. It exposes your immune technology to an artificially weakened virus, resulting in a lazy adaptive response from your immune technology. In much the same way that your leg muscles atrophy if you stop walking, your immune system begins to weaken if you don't exercise it. And this leads to an increased risk of being unable to defend against future exposure to infectious disease, which is exactly what we're seeing with this Canadian study.
