Will my student insurance in US, as an international student cover my pre-existing ileostomy caused due to Crohn's disease?
You will find that insurance in the US isn't nearly as comprehensive as that of many other countries. Neither is there any universal health system in the US. So, this is a question that you need to explore separately with each university you might attend. 

My best guess, however, is that you will be disappointed most places. In the absence of clear statements from individual universities, assume the worst. 

Europe, including some non EU countries have good coverage, but even there you need to know whether it is available to non-citizen residents and students. It will vary, but the chances there seem better. 

But, to be clear, the health care system is quite good: doctors and such. It is the health insurance system that is brutal, and the two are quite distinct.