There is perhaps an intuitive sense that human life is protected by the odds. It seems reasonable that the risk of cataclysm is sufficiently small to have enabled human beings to thrive for millennia, and expect to do so for many more. Most commonly perceived threats are associated with vanishingly small likelihood of occurrence. A worldwide extinction such as that which befell the dinosaurs is likely rare enough to have been considered a one off. This perception applies to most conventional threats, such as a catastrophic asteroid impact or nuclear calamity. The odds, simply, appear he to be in our favor.
It may however be the case that there are mortal threats lurking in far more likely scenarios. To take one such example, it is reasonable to assume that there is at least one, and likely several amino acid sequences that would code for a pathogen that would be devastating to human life. Imagine, for example if the human immunodeficiency virus had a genetic makeup that would allow it to be passed by casual contact, or to be transmitted with the ease of the common cold. It is possible to conjecture the existence of such a genome and wonder how likely random mutation is to produce it. Is such a peril thirty base pairs away? ten thousand? A million? It is also possible to consider that such a fateful genetic sequence might be the result of human manipulation.
This is not to imply that human beings are necessarily doomed, or that a biological catastrophe will wipe out the species. It is to suggest however that it might not be unreasonable to wonder what the true odds of disaster are, and when luck might run out.