The worst part is how the diseases from other animals are way too deadly to humans. The human-specific diseases like the common cold are a great example of a d

The worst part is how the diseases from other animals are way too deadly to humans. The human-specific diseases like the common cold are a great example of a disease that is ideally adapted to its host. Its high adaptable (thus getting around our immune system), it spreads effectively, and crucially, it's very rarely fatal. Because it's not deadly, it can easily infect the same host over a hundred times throughout its lifetime. Viruses that are routinely deadly to the organisms catching them are not adapted well to their hosts. We see this with HIV (came from monkeys), the worse flus (came from birds), and all sorts of other stuff.