The probability of having autism is greater for an individual if it runs in their family. This suggests hereditary factors. But what do you actually inherit? What you inherit is a predisposition for a specific development of your brain. This, together with what you experience while growing up, can lead to the development of a mild to severe form of autism.
Disease can be seen as the antechamber of death.
Are there reliable tests for predicting disease? The short answer is: not for Alzheimer’s or other common diseases. Tests do exist for some specific diseases, most with known familial predispositions, such as Huntington’s disease, cystic fibrosis, Duchenne’s dystrophy and many more.