Why do viruses mutate?
Viruses can spread and mutate during fertilization. The virus mutates in only the egg or sperm cell so when the cell starts to reproduce it will be infected with the virus.
The only way a virus can reproduce is through a host cell, which it does by attaching its surface proteins to the cell's membrane and injecting its genetic material into the cell.
This genetic material (either DNA or RNA) then carries with it the instructions to the cell's machinery to make more viruses.
These new viruses then leave the cell and spread to other parts of the host organism.