Without going through all of the philosophical tangles, diseases have various kind of existence. There are the definitely really real diseases, like kidney stones or tooth decay. These are unambiguous physical entities that just about everybody would rather not have. Then there are diseases in which we don't feel there's anything wrong until a doctor tells us to worry, like high LDL cholesterol. There are other diseases we aren't actually sure constitute specific physical states or processes or single entities, even though there are a lot of people out there who have similar clusters of complaints, such as fibromyalgia and chronic fatigue syndrome.
Then there are mental disorders such as one of our all-time favorite topics here, Attention Deficit Hyperactivity Disorder. The diagnostic criteria for this disease include the following kinds of observations:
Often fails to give attention to details or makes careless mistakes in school work, work or other activities;
Often avoids, dislikes, or is reluctant to engage in tasks that require sustained mental effort . . . ;
Often loses things necessary for tasks or activities;
Etc. Then there's another set of criteria -- either one will do, there are two ways you can have this disease:
Often fidgets with hands or feet or squirms in seat;
Often runs about or climbs excessively in situations in which it is inappropriate;
Is often "on the go" or often acts as if "driven by a motor";
Etc. I don't know about you, but it appears that I have the first version of the disease, at least by most Thursdays, and I tend to get the second version during baseball season. I'm not sure how often is "often" enough though.
Children who are diagnosed with this disease are usually prescribed amphetamines, which are also sold illegally on the street to addicts.
CDC has recently released results of a study based on the National Survey of Children's Health, done in 2003. It turns out that a child's chances of having this disease are 8.6% if the parents speak English, and 1.3% if they don't. The chances are 8.1% for children who have health insurance, and 4.9% if they don't. More than 11% of children in Alabama have the disease, but only about 5% of children in Colorado. (Must be that bracing mountain air.) So this is a disease which is caused by having English speaking parents, having health insurance, and living in a humid climate. Weird etiology, I must say.