One argument within the wider debate about genetic engineering and genetic modification is that it is a speeding up of the process of natural selection. This means that what would take millions of years in terms of evolution, scientists and test tubes can manage and achieve. But that is surely the point of the problem. What takes place in nature is a holistic process that includes all of the possible variables. What takes place in a laboratory holds, by definition, all other factors equal, but nature does not hold them equal. Every action has a consequence.
People know that the human body is very complex and, in a general sense, can be said to be made up of a combination of good and bad genes. The basis of genetic engineering is that the bad genes can be altered so that these no longer pose a threat to the well-being of their owner. The technology exists to do this at the level of sperms and eggs but most governments see this as unacceptable.
Genetic engineering uses a viral vector to carry the functional -- replacement -- genes into the human body. However, two problems result. The first is that the effect that these viral genes may have on the human body is not yet known. The second is that it is not yet known where the functional genes will be placed, so they may replace genes other than the mutated ones. This could cause other as yet unknown illnesses.
If all defective genes are replaced with functional ones, future generations will all have the same genome. This will mean that while on the one hand everybody will be immune to existing illnesses, everybody will be equally susceptible to as yet unknown diseases or viruses. In the worst-case scenario, this could lead to the extinction of humans.