IBDs like Crohn's and Ulcerative Colitis have increased in incidence (controlling for better diagnosis) as countries develop[1] [2]. People in developing countries have a lower rate of incidence, and if they move to developed countries, the incidence remains low. However, their children, born in developed countries, experience the higher rate of incidence. Something about being a child in a developed country increases your chance of developing an IBD
Several theories have been proposed, including hygiene hypothesis, lack of fiber in the diet, and pollution, but finding causality has been hard. Worse yet, nobody knows what causes these illnesses.
It's also interesting to note many food allergies have increased in prevalence in the last 30 years - although the causes for that might be different, they are both GI related auto-immune conditions.
[1] https://www.gastrojournal.org/article/S0016-5085%2811%290016...
[2] https://www.thelancet.com/cms/attachment/1bc07ee3-782c-4670-...