It seems like when we observe animals in the wild, the occurrence of noticeable disease in adult individuals is much lower than in humans. Why?
There are a number of reasons that this could happen, but I want to know which reasons contribute the most, or what other reasons I haven't thought of could be involved. So, here's my list of theories:
1) Animals that are prone to disease usually die when very young, while humans in developed countries are given vaccinations/medical attention and have a much greater chance to live to adulthood and advanced age. However, even taking this into consideration, it seems like human populations in third-world countries with little or no access to medicine are much, much more riddled with disease than animal populations appear to be. (Or do they only look that way because that's what we see the most of in the news?)
2) The animals that we see in documentaries are generally purposely selected for viewing as being healthy individuals or a mostly healthy group. This doesn't account for YouTube's many amateur safari videos and pictures, however, where it still seems like animals die much more from territorial disputes or preying than of disease. Safari-goers aren't likely to sit around filming diseased animals, but it still seems like animals 'in the background' and in herds appear to generally be very healthy when it'd be pretty easy to notice a somehow diseased or disabled person in an equivalently large group on the sidewalk.
3) Most humans don't know what common wildlife diseases look or sound like, and at a distance they all appear healthy to us even if there is a roughly equivalent percentage of disease among adult animals as among adult humans.
3a) Animals are much better at concealing the symptoms of their common diseases than the average human, as a defense mechanism from predators. (I know that cats are particularly good at not showing that they're in pain, but I'm not sure how many other animals behave this way.)
4) While most modern humans go out of their way to do every possible thing they can to make sure an infant survives, animal parents purposely kill, abandon or even eat young that they can sense are diseased or disabled in some way. I saw an example of this on a youtube video where a lion cub was gored by a cape buffalo and its back legs were paralyzed. After numerous failed attempts to nudge the cub into standing or walking properly, the mother purposely killed the cub with a suffocating bite and ate it. (Did/do humans practice this to any significant extent? I once read about a culture, can't remember which, that didn't consider an infant viable enough to even name until it was at least 3 months old. I also read that either the ancient Greeks or the Romans would bathe a newborn in wine, and if it survived, it was deemed strong enough to deserve a chance at life. I don't know how true either of these claims are, though. But that's kind of a different question, so consider it optional 'bonus points' to answer.)
5) Diseased and disabled animals are killed off by predators or rivals quickly enough that the populations we see are filled with healthy animals just because the infirm are constantly weeded out. This doesn't address animals that have few/no natural predators or rivals, such as in the case of areas that have so many deer with no natural predators that human hunters must cull them to keep them from reproducing out of control. Are these populations more prone to adulthood diseases than populations that have natural predators?
6) At about seven billion, humans are easily the most populous species that is the size of humans. Other animals with larger populations are much smaller than humans. Compared to other animals of our size or larger, we are intensely overcrowded, leading to lots of people coming in contact with lots of other people all the time, frequently on an international scale as thousands of planes and ships travel around the world on a daily basis. This only covers contagious diseases, though. Even the most isolated hermit could die from a genetic condition that he had since before he was born.
7) Stress. Animals live what we would consider to be a very high-stress life, but they experience their stress in short, intense bursts. Humans, even (especially!) those who live in first-world countries with lots of information to learn and lots of things to do, often get to the point where they can't figure out where the 'off' switch for their stress is. Stress is a silent, gradual killer, and it's possible that minor genetic conditions that may have otherwise been fairly benign or unnoticed turn ugly as a person ages through a life where stress is constant.
8) Diet. It's no secret that there's a lot of heart disease and other diet-related issues in the modern world, but I don't know if that covers ancient humans, and there seems to have been plenty of disease to go around in the olden days even when people ate purely natural foods.
I can't think of any other theories at the moment. One way I thought of to possibly help answer this question was to take into consideration domestic animals - how many diseases do veterinarians have to treat compared to how many diseases human doctors have to treat? This line of thought is rather tainted by the fact that domestic animals are often given vaccinations of their own, though, but it doesn't seem like they receive nearly as many as humans do.
This is my first time asking a question on stackexchange, so I hope I've given you plenty of meat to chew on with this post. I look forward to the replies!