Viral diseases most definitely can be cured with current research techniques. If anything, they present a much easier target than cancer or other end-of-life diseases, because viruses are comparatively simple little infectious agents and comprise the sole cause of the disease, so a cure only needs to attack one thing. Their genome is on the order of a few thousand base pairs, usually only 10-100 genes. You should check out all the new antiviral drugs that have been developed over the last decade, it is really the product of the new age of genome-level research. [1] The main problem in developing antiviral drugs is toxicity, because viruses co-opt certain parts of the cell and have proteins that are usually similar to normal human genes, and you have to find treatments that only target the viral activity. For example, the first commercially successful antiviral [2] targets reverse transcriptase, which performs a function that normal human cells never do (transcribe RNA -> DNA).

[1] http://en.wikipedia.org/wiki/Antiviral_drug

[2] http://en.wikipedia.org/wiki/Acyclovir

Some viruses are better than others at staying around. Retroviruses, such as HIV, have an enzyme called integrase that adds the virus' genes into your DNA. That makes it particularly difficult to get rid of. Most viruses aren't that good.

So, it's entirely possible that the rogue code remains unactivated in his cells but remain rather ineffective because the white cells come from a different lineage due to the transplant.

Funnily enough Retroviruses are also the major vector in gene therapy.

Retroviruses incorporate into the host genome and is important for gene therapy. This allows for a long lasting effect and is essential depending on cell type. Different cell types have different turn-around rates. If the gene of interest is for a protein, you need incorporated DNA for continual expression.

Don't forget that you can use viruses to kill bacteria:

http://en.wikipedia.org/wiki/Phage_therapy

Yes, viruses are both terrifying and awesome :)

That isn't accurate.

The body frequently cures itself of viral diseases.

Do you mean can doctors cure a viral disease? That too isn't really accurate, since what they did is get the body to cure itself - which isn't that different (in principle) from a vaccination (although it's a massive difference in practice).