I operate under the assumption that I'm going to have every medical problem - everything my ancestors have died of or even been diagnosed with, everything my genetics or behaviour makes it more likely for me to develop, basically everything I have an even slightly greater than average likelihood of contracting. I assume I'm going to get Alzheimer's, I assume I'm going to get cancer, I assume I'm going to get Barrett's esophagus, I assume I'm going to break a hip and become helpless.
There are people who think this approach is needlessly pessimistic, and who say things like "Why don't you just adopt a healthier lifestyle so you don't end up with these health problems?"
Today my shower gave me an analogy to explain why:
Suppose you want to enter a university program, but you don't have enough money to pay the tuition.
The university offers a scholarship to the very top student in the whole university every year, and this scholarship offers enough money to pay for the whole program. But only one such scholarship is offered, and it's only available to the one student with the very top marks in the whole university.
So is it a good strategy to decide "Okay, problem solved. I'll just get the best marks in the whole university and pay my tuition with that scholarship"? Or would it perhaps be a better idea to assume you won't win the big scholarship, and instead work out a way to assemble the funding from other, smaller, more winnable scholarships with multiple recipients, combined with perhaps a part-time job and some student loans?
To win the one single big scholarship, you have to address and overcome a wide variety of ever-changing factors. You not only have to be at the top of all your classes, you also have to be aware of what kind of marks other students are getting in other classes and figure out ways to top them. You need to keep in mind how various courses are graded, and choose courses (and maybe even a major) that make it more possible to get higher marks. (For example, it's easier to get extremely high marks in a math class than in a literature class, because answers to math problems can be unquestionably and objectively correct, whereas a literary analysis essay is more subjective and far less likely to be interpreted as perfect and therefore worth of a 100%.) You also have to be able to read your profs to determine how to extract the highest marks from them, (for example, I've had profs who give higher marks to essays that prove conventional theses, and I've had profs who give higher marks to essays that take a risk and prove an unconventional thesis, or have a good go at disproving a conventional thesis), and you have to do this early enough in the course so as not to have a sacrificial first assignment. To say nothing of the stress you'd have to put yourself under and the pleasures of life you'd have to give up to study enough to earn top marks in all things at all times!
This is rather difficult, isn't it? In addition to doing your absolute best in everything at all times, you have to be constantly and at every moment on top of an ever-changing lineup of factors, many of which are completely beyond your control. And if you drop the ball even for a second, there goes the scholarship you were depending on for funding. It's a lot easier, less stressful and more feasible to operate under the assumption that you're not going to get the big scholarship and instead work out a way to get more predictable funding. If you get the scholarship, bonus! All your problems are solved! But if you don't get it, you're prepared for the eventuality.
Similarly, I find the list of things you're supposed to do to prevent Alzheimer's, cancer etc. is large, complex, overwhelming, and ever-evolving. There is contradictory information out there, some of which is actively trying to discredit each other. It encompasses every facet of life, some of it involves factors that are beyond our control, and much more of it involves factors that it is possible to control but very difficult to do. There are aspects of it that we don't know yet, and there are aspects of it that may be thought to be helpful but later be discovered to be harmful.
So instead of making myself a slave to all that, I just assume I'm going to get all these diseases and plan accordingly. If I don't get them and end up dying peacefully in my sleep, bonus! But if I do get them, then I'm prepared for the eventuality.