And we know about giardia. It has been customary for people to assume that when we are not directly harming wildlife, we are not harming them. Also, they often have much greater visual and auditory acuity than we do, and hence can be disturbed by sensations that we wouldn't even notice. Technology has made it much easier for people to reach wildlife habitat, including areas where access used to be difficult, such as cliff faces, caves, under water, and inhospitable climates. That and increasing population have squeezed and frightened wildlife out of its preferred habitat, both temporarily and permanently, depriving it of needed foods, shelter, and choice of mates.
Speed is a big factor.
Being encased in a motor vehicle greatly reduces the sensations that you experience. This is one reason why mountain bikers are never satisfied, no matter how many trails they have access to, and why they are exerting such tremendous pressure on land managers around the world to give them ever greater access. He told me that no one would ever want to bike Mt. Everest. The movement of the parent, or sometimes the odor of the human, can direct predators to the nest, causing the death of some of the young. Sometimes the parent in its rush steps on an egg or knocks it or its young out of the nest, leading to certain death for the offspring.