The number one threat to humans in terms of unknown and undiscovered infectious diseases are animals that live in habitats like dense forests and, quote, wild and remote places out of reach of humans. Under ideal circumstances, the animals, like certain bats that carry diseases and humans... They would never make contact with most of them under normal circumstances, and there would be very small risks in terms of them infecting people and spreading to human hosts, whatever they're carrying. However, there are a couple trends that have been happening that affect things that make the world we live in not ideal. The first is that we're engaging in massive deforestation around the world, seeking to turn wild land into areas that we can use to make money. The second is climate change, making previous habitats inhospitable to animals, causing them to move, like to seek out new sources of water and food or just good living. You saw a wolf in Chicago! I did. And sometimes because of these moves and these migrations, they end up closer to humans. These two forces combine to increase the chances that animals that could carry new and novel infectious diseases will be in closer proximity to human settlements and also increase the likelihood of them infecting a person and causing an outbreak.