Some mutations lead to benefits such as increased immune health or play an important role in evolution, while others cause serious health problems, like cancer. Many mutations occur in noncoding DNA and are completely neutral.
A mutation is a permanent change to DNA. These changes occur when DNA repair mechanisms fail to repair damage, when DNA replicates incorrectly, or from the deletion or insertion of DNA segments. Some mutations occur within the lifetime of a single organism. These somatic mutations involve direct damage to an organism's DNA from an environmental source. For example, damage from ultraviolet rays damages DNA in a way that gives rise to cancerous cells.
Other mutations are heritable and pass from one generation to another. One particular heritable mutation causes a deletion of 32 base pairs affecting the human CCR5 protein. Research suggests that people with this gene possess immunity to bubonic plague or smallpox, a very beneficial mutation to people at various periods in human history. Because of higher survival rates of people with the CCR5 mutation, the mutation persists into the present day. For modern humans, a CCR5 mutation means resistance to HIV.
Mistakes occur constantly during the replication, translation and transcription of DNA. Fortunately, nearly 98 percent of human DNA is noncoding DNA with no apparent function and mutations in noncoding DNA have no effect.