Stanford has a free introduction course. There is also edX.

Now, I’ve only taken an introduction into cybersecurity at a graduate level, and I will say it’s a difficult career path to master. I’m not saying that to dissuade you, but rather give you a reasonable expectation of the level of effort. But it’s a super fascinating area of computing, and it’s certainly not going away any time soon.

Edit: Oh, and I forgot to mention Bioinfomatics where you could put your existing medical knowledge to use in a computing career.