New infectious diseases appear mainly as a result of chance mutations in pathogen genomes?
Mutations happen all the time when DNA is replicated. These random mutations are the building blocks for evolution under natural selection. So when a single nucleotide is changed, (or deleted or added), there are three possible outcomes. 1. the mutation is neutral, so nothing happens. 2. The mutation is lethal or mal-adaptive for the organism, in which case natural selection will remove the individuals carrying the mutation from the population, or 3. the mutation results in some change in function that helps the organism compete under natural selection. This 3rd kind is the rarest, but with pathogenic viruses and bacteria, with huge populations, the evolutionary test pool is much larger because there are so many new mutations being put to the test with every generation. So, for example, a mutation in a pathogen adapted to spread through chickens, could change the pathogen to be better adapted for spreading through humans.