Drie35

Hi, it is depends of what kind of illness she has. But I don't think it would be a basis for Visa denial. A delay maybe. 
I petitioned my mother last year January 2017. She was sick during the petition. I would like here to come over, so I can take care of her. I can't just go home there and stay there. I have to work. I am the bread winner. So, regarding her illness, 2 doctors diagnosed it as lung cancer but we don't know what stage because my mother does not want to do the biopsy. The nodules are spreading very fast. Fast forward, after the case closed, we received an interview date for January 2018. She did her medical test to find out she had a severe tuberculosis. Doctors even told her that she might not able to come here in the US because of the severity of her illness. She is skin and bone. It was depressing. As a normal procedure she had to undergo 8 months treatment. Thanks to God, the antibiotics they gave her was working for the first month she had started taking it. On the second month of treatment, she stopped coughing and her appetite to eat is back. She gain weight each day. She has to complete the months of treatment and completed it on Sept. 25. So, everything went well. She is cured, passed her visa interview, done with PDOS and booked her ticket to arrived here before thanksgiving. I am sharing this to you, to encourage you, that whatever illness she might have, DON't lost HOPE. Keep your FAITH knowing that God is in control. God bless. 
 

Please consider the impact of emotional stress on Mom's illness. It almost always makes sickness worse. As most of the posters here would agree, immigration is one of the most stressful life experiences, especially when there is a major culture change. 

Be sure to consider that finding someone willing to take her on as a patient may be difficult.  A doctor is not required to accept a new person into his/her practice.  The wait time for a new patient appointment in my city is 6 to 8 months if you want a reasonably well qualified individual; it may be half that for someone educated at a no-name  institution.  Treatment in the ER for a chronic condition is not possible and the copays for an ER visit on our plans are $1500 per visit.
 
Nursing home care is NOT covered.  Neither is in home health for other than very specific conditions. Count on a sitter through an agency so you can work of more than $20 per hour; these are not medical professionals but simply sitters. 
 
Have a reality check. My recent 5 day hospitalization without any surgery and 2 week nursing home stay for rehab was billed at $65,000 and bills are still coming in.
 
Your situation is the reason many oppose allowing parents to be brought to the US. 

If your parents are already here in the USA, you can submit the forms necessary for Adjustment of Status. They will not be able to leave the US until they get Advanced Parole. Health insurance is usually an issue in these cases, especially with elderly parents.
 
One advantage of filing the I-130 from their home country is that they have time to get their affairs in order and prepare for a life in the US.

Take a look a these link:
https://www.visajourney.com/content/immigration-parents
https://www.uscis.gov/family/family-us-citizens/bringing-parents-live-united-states-permanent-residents
BTW,AOS on tourist visa is not recommend.
Good luck!