It's funny the subject of immune systems came up. A recent study (yeah, I know...a study shows what you want it to) showed that children on farms, and children who were exposed to dirtier environments got sick less than children who were raised in "sterile like" homes.
I totally believe this! Your body has to BUILD a good immune system. It can't do that if mom is cleaning baby off every time it gets close to dirt.
My sisters second child was like that. She didn't want her "dirty", she didn't take her out in the cold or rain, etc. She developed asthma, and gets sick VERY easily now.
My daughter is total tomboy. (Our girls are 10 now...2 months apart). Mine has always loved to play in the dirt, mud, rain...yes she has eaten dirt
My daughter has been sick enough to go to the doctor only a couple of times when she caught strept throat from her half-brother. She never even had 1 ear infection....great immune system.