Immunity does not build. It's more akin to blood oxygenation level in the sense that it's either strong or weak at a specific point in time. You may have the correct types of white blood cells to attack certain viruses at certain times but you can't get the genetic code for all viruses. Immunity doesn't last forever. If "building one's immunity" were true, older people would have more sophisticated immune systems, but they don't. This is an incorrect idea perpetuated by the uneducated side of the medical establishment (mostly nurses) and even teachers who are commonly around germ magnet kids.

Every time you get "sick", your body is damaged. Most parts of it regenerate but some things get permanently damaged. For example, the Wuham viruses can permanently damage your lungs even if you survive it. The human immune system was created to survive a local existence, not to be a global petri dish. There's simply too many pathogens for your body to successfully defeat over the course of a lifetime.