Everyone kind of views the human body as balanced on a peak of health and anything that disrupts that the body falls to illness - the truth is that it is more like the body is down in a valley and it takes something pretty extreme to overcome its normal homeostasis. (sorry, that might be a confusing analogy sigh).
There are RNA (transcriptase?) molecules whose job it is to go over the DNA and correct transcription errors. We contain all the DNA from our origins with all the DNA that makes us different from pond scum or primates. It is just that certain genes are 'turned on' (crap I miss my ex - she was a plant phys. whis and could get me closer) or 'expressed'.
People sometimes wonder why no one has blue hair and why no mammals have blue fur - the reason might be that the combination of genes that would express blue hair cause other issues that are deadly to mammals. Attributes that we consider beneficial (surviving malaria) in a different environment proves to be detrimental (cycle cell).
But I digress