More people = More Illness?

[Stravo]

Is it a natural presumption to believe that as the human population grows so will people with illnesses, in particular genetic defects like autism? So when reports come out saying things like "Mysterious rise in autism cases" that part of the reason my simply be there are more people in the world?

[SirNitram]

Three-part reason.

1) Greater sensitivity of tests. We are now aware you can be functionally autistic and that it's part of the diseases. We similarly know that someone keeling over of a bad liver at 50 might have Hemachromatosis.

2) Greater tolerance. A genetic disease wipes itself out if it's victim can't reproduce. But modern science means these people can live fuller lives and potentially have offspring.

3) Greater population. This means you have a greater resource base for diseases to evolve and find new niches in, and means to exploit.

[Broomstick]

You also have to be wary of attributing "genetic defects" to just one factor.

There are some conditions - sickle cell anemia being one, but not the only one - where the benefit to the population conferred by having large numbers of people with one affected gene outweighs the early death of those with a double-dose, given the right environment. Or hemochromatosis, which in an area where the diet is iron-poor is actually an advantage in the Darwinian sense, and only become a liability when iron content of the diet improves. Environment DOES count, even today.

Autism is a different animal - we don't know what causes, or even that there is just one cause. There may be several forms of autism, each with its own cause, but all displaying similar syptoms. Or it may not be a "defect" so much as an extreme shuffling of genes - many autistic traits are found in highly successful, very social people. It may be that having a "half dose" is beneficial, leading to high intelligence and intellectural capabilities but a "full dose" is detrimental... which might account for the spike in cases in places like Silicon Valley were highly intelligent people work, meet, mate, and reproduce. Back in the old days when mating choices were more limited and mobility considerably less, the highly intelligent didn't marry other highly intelligent people quite so often, and this might have been a good thing.

In theory, large numbers of people should reduce the number of genetic defects as a percentage of the population because inbreeding is less likely... but the absolute numbers of a syndrome/disease/illness may go up simply because there are more people overall.

Some diseases are crowd diseases - they require concentrated populations to sustain themselves. Such as measles, flu, and so on. I think 100,000 people is the cut-off for measles... below that number the disease can't sustain itself, it runs out of new victims. And the larger and more concentrated the population the more such crowd diseases you can have.

[Admiral Valdemar]

A simpler problem arises from more people acting as breeding grounds for more infectious diseases and the fact that people explore and live where they want which is how many emerging diseases come about.

[Mayabird]

For some diseases also (particularly those that strike the elderly) people just didn't live long to get them, or if they did, they were considered just dead of old age. If you died at thirty-five from overwork, you couldn't get cancer at fifty or Alzheimer's at sixty. Die of diabetes at forty? Well, you were just old, and lots of people die at forty.