Well I'm kind of in the same boat as you but I just finished a CNA course this fall semester and I'm waiting to take physiology. I took my course alongside microbio and honestly it was doable but I was going to school 4-5 days a week for around 2 months til my CNA course ended. It's doable but getting a good grade in your bio classes is absolutely crucial. That being said, summer semesters are generally shorter and at my college it's kind of hit or miss as whether that shortness is a good thing. Fewer assignments maybe, but less time to learn the subject. As to how much more difficult it would be, CNA courses aren't exactly hard per se. They're time consuming and require dedication, but written work is easy, clinicals don't really require much in the way of brains, and the certification test is easy if you practice and study. The problem is that the more time you are taking your CNA course, the less time you have to focus on physiology. If you think you can handle it, go for it. I know exactly how it feels to be struggling to finish your prerequisites in time for application deadlines so you don't have to wait another whole year. Best of luck. On a sidenote, is it absolutely necessary for you to take your CNA course in the summer? I would personally see if you can get training at a nursing facility this spring so you don't have to double up.