Mutations occur when the DNA replicates or makes identical copies of itself. If during replication substitution, deletion, or insertion of nucleotides happen then that gene is mutated. Mutations are not very common in humans and occur with a very low frequency since human DNA has the capability of correcting many of these changes. For comparison, viruses mutate very quickly since they lack the proofreading mechanisms and infect us with unpleasant and new diseases every year.
Harmful mutations are passed on from one generation to the next hence they continue to cause health problems. Specific ethnic groups accumulate more mutations through inbreeding (having offspring with the members of a small population). In these cases the frequency of harmful mutations is dramatically larger than a population with random mating. A good example is the US with a large and random mating population versus a small and nonrandom mating population.