It's actually a lot more interesting than what the article explains here. The immune system essentially goes through a process of hypermutation and can be compared to a hashmap which is populated during the first few years of one's life. After that the hashmap is essentially frozen. This is why early immunization is very important because being exposed to mild pathogens when young helps one's immune system recognize and deal with similar pathogens later in life. The immune system only gets worse over time which is why eventually most viral infections overwhelm the immune system and either cause death or cancer.