The most destructive mutations; the ones causing permanent genetic diseases occur at the germ line level of humans.
Somatic mutations occur all over the body and normally are not transmittable to the zygotes. Normally female egg cells are protected from being mutated and sperm can be mutated but not likely.
We now have 18,000 (fertility doctors test for these) known genetic diseases from this process of mutagenic pathogens at the germ line level of reproduction. This mutagenic pathogen phenomenon is the main cause of human suffering an is the cause of most genetic losses. In rare cases it is radiation causing a mutation but not solar radiation because of the location of reproduction inside the female body, protected from solar radiation.
What takes place is a mutagen pathogen, HIV, HSV1, HSV2, HPV, and others will disrupt the cell replication at a time where there is absolutely no defenses (no disinfectant in the female) and no immune cells to stop these, incorrectly formed, cells from replication. And viruses enter these cells and perform an event called reverse transcriptase. In this process the virus infuses its molecular proteins into the DNA of cell and changes it forever. The types of mutations found in humans are shown in this video from the USA National Library of Medicine.
Since these are the first cells of the infant, they just keep replicating with this new deleterious mutation.
There has never been found a beneficial mutation in all of the history of the Human Genome project, nor in any of the studies done by research doctors and MD’s.
These pathogens are the reason for the 18,000 genetic diseases and the fact that 1 in 9 babies in the USA (NORD) are born with a serious genetic disease, requiring life long medical treatments in order to live even a half way decent life.
This is how the process is caused:
