A great example: It's been discovered that fatty tissue in individuals with obesity often have a very high number of macrophages - white blood cells - that cause a general inflammatory response, in fact a response very similar or identical to what the body would do in the case of a parasite. (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC297006/)
It's also well supported that being exposed to allergens at a young age decreases your likelihood of being allergic to them later in life, as the body has time to develop an appropriate immune response while antibody production is still malleable. I.e. A child who has never touched a cat is more likely to be allergic as an adult than one who grew up with cats.
I would argue that the biggest and most effective change that saw an increase in general health and longevity in the 20th century was the widespread use of sanitary cooking and waste management. The ability to stop excrement from contaminating drinking water and food was responsible for a huge reduction in disease and especially spread of parasites.