Edit: I thought you were talking about being tired of admittimg URIs. Then I saw your title "has your immune system improved." Makes total sense. I hope it does get better because it sucks.
I have the immune system of a god after a three year peds residency. There isn't a virus I haven't encountered, and that includes Ebola, both HIVs, hepatitis A-F (discovered that last one myself), and the T-virus from Resident Evil. Immune to 'em all.
1) Vitamin D supplementation. There is a pretty good evidence base that this is the one vitamin supplement that actually reduces the rate of URIs.
2) Universal gloving. The studies on universal gloving were mostly done to measure the rates of nosocomial infections, but the results were pretty dramatic: if you never touch a kid without gloves (whether or not they look sick) there is a significant drop in the rate of nosocomial infections. I don't think its a leap to say that the same practice is also protective for you. Also I haven't been falsely accused of not washing my hands by a confrontational patient since I began universally gloving, which for me is reason enough to do it.
3) Better sleep. OK, as a resident you can't act on this (unless you have sleep apnea that you can correct) but there is very strongly evidence that the less sleep you get, the more susceptible to infections you are. That's one of the reasons that attendings get fewer infections that residents. Just another reason to prioritize sleep in the spare time you have.
I do think that you do build up some immunity with repeated exposures as well, and you also just get better at positioning patients to avoid exposures (examine the patient in the parents lap, with their side towards you, so that the aren't pointed at you when they cough, etc).