Normal is Relative

I was just pondering recently about how in some situations, a certain condition could be viewed as genetic, whereas in others it's environmental - but it's still the same condition. An example is AIDS. AIDS is a viral condition, right? It's caused by HIV - human immunodeficiency virus.
But if there were chimpanzee doctors, they'd call it a genetic condition. Some people, and most chimpanzees, are genetically immune to AIDS. They can be HIV or SIV (simian immunodeficiency virus) positive, but they will not get the immune problems that characterize AIDS. Almost all chimpanzees have SIV, and almost all are immune to it. So in order for a chimpanzee to get AIDS, they have to have the genetic susceptibility to it.
Phenylketonuria (PKU) is another example. PKU is a condition in which a person can't metabolize phenylalanine. If they have a low phenylalanine diet they are (mostly*) normal. With a high phenylalanine diet, they are developmentally delayed, with behavioral differences including autism and hyperactivity, and have a pale complexion and a distinctive smell.
In our society, where most people can metabolize phenylalanine and eat a high phenylalanine diet, (untreated) PKU is a genetic condition. However, if there was a society where most people had the genetic inability to metabolize phenylalanine as well as a low phenylalanine diet, then it would be considered a dietary problem.
Incidentally, that's the case with scurvy. Many species, such as rats**, can create their own Vitamin C, but humans (as well as guinea pigs) can't make our own Vitamin C. Therefore, our only source of Vitamin C is our diet, and most people eat a diet with enough fresh fruit and such that we don't get Vitamin C deficiency (scurvy). If a person gets scurvy, it's because of their diet. But a rat who has Vitamin C deficiency would probably have some kind of genetic abnormality that means they can't make their own Vitamin C.
Basically, in terms of the factors that cause a condition, doctors look for what's different. What is different about this person, that xe is having repeated pneumocystis pneumonia? HIV. What is different about this kid who's developmentally delayed and has an odd smell?They have two mutated alleles in a certain gene. Those are considered the cause, not genetic susceptibility to AIDS or a high phenylalanine diet, because that's what is different.

Ettina

* However, see Inhibitory Control in Children With Phenylketonuria, State Regulation and Response Inhibition in children with ADHD and children with early- and continuously treated phenylketonuria and School Performance in Early and Continuously Treated Phenylketonuria.
** When I told my Dad this, he asked if sailors wouldn't get scurvy if they'd eaten raw rats (cooking destroys Vitamin C). I'm not sure if that would work, but it's really funny to imagine.
[Edit: Actually, there is not enough vitamin C in rats. You would need to eat an immense amount of rats to get enough vitamin C, and it's unlikely a person could eat that many rats. I suppose if you had some way of removing most of the non-vitamin C containing parts, it might work, but that might be harder than simply bringing citrus fruits along.]