How can genetic engineering be used to treat or cure diseases?

Genetic engineering is the mutation of a person's genes to meet certain needs/ demands. Most pre-mature babies have quite a number of defects. For example, a friend of mine was born 14 weeks old and he has an extra muscle growing in the nose which causes him to have runny nose for the rest of his life and a bone defect in the knee which means he can never run properly for his whole life. Genetic engineering may be able to change that. By changing certain genes, he may never have been born pre-mature, preventing him from diseases. Another way genetic engineering can be used is to modify the genes of someone infected with a certain disease. This may cure many rampant disease and prevent pandemics such as the current situation. However, it is very costly and not everyone may be able to afford it. This means that only the rich will have access to such medication if genetic engineering becomes something to the public.