This applies not only to Alzheimer's disease. There are other diseases. We know of one gene that makes you susceptible to Alzheimer's. It's a simple test, but you couldn't just go to a hospital and get it. Your doctor might be able to get if for you if he thought there were reasons why he would like to be sure that you are or are not on the way to getting Alzheimer's. It's called the ApoE4 test. This is a question that really hasn't been discussed as openly as it should.
I believe in the States, if you go into a hospital, they routinely test for this gene—but they don't tell the patient, it's buried in the records—because it might help the clinician make a diagnosis subsequently.