BTW, you do not “build up your immunity” by getting sick. Neither do your kids; getting sick a lot in daycare as a kid will not make you get sick less often as an adult. You can develop immunity to specific diseases, but there are too many diseases out there and they mutate too rapidly to develop immunity to enough diseases to end up generally less prone to disease.
Getting sick actually makes you more likely to get sick, not less. Take measles:
“A new epidemiological study suggests that [people who’ve had the measles] remain susceptible to other infections for more than 2 years, much longer than researchers anticipated. The results bolster a hypothesis that the measles virus undermines the immune system’s memory—and indicate that the measles vaccine protects against other deadly diseases as well.
Researchers have long known that measles inhibits the immune system, but they generally thought this effect wore off after a few months at the most. However, studies of children in developing countries, where most cases occur, found that measles vaccination reduces the overall death rate from infections for up to 5 years, suggesting that preventing the disease somehow provides protection against other illnesses.” —Mitch Leslie, Science/AAAS News
Not so marvelous now, are they?
Honestly, if getting sick made you less likely to get sick afterwards, everyone who got Smallpox would have had the immune system of doom and never died of anything; malaria would have rendered sub-Saharan Africa a disease-free zone.
Long term, populations get less prone to getting specific diseases just because the most susceptible people have died and people with mutations or behaviors that make them more resistant have survived.
I have been reading a lot about AIDS.