I need a contagious disease that will alter DNA to change how the human body will function.
Thoughts?
The most famous retrovirus is HIV. It attaches to a protein on our white blood cells that allows it to enter the cell without being enveloped and destroyed. On a side note, some people are resistant or immune to HIV because of mutations in this surface protein. The protein is vestigial and not used by the white blood cells for any function. If it is mutated, the HIV cannot attach and enter a white blood cell - making someone immune.
I believe it is not the class of viruses (retroviruses) that you are interested in so much as the behavior. You appear to be looking for something that changes DNA. Bacteriophages are a retrovirus of sorts when looking at behavioral aspects.
They insert their genome into RNA and the cell either begins decoding it to make virus proteins, or this RNA sits in the genome and the infection becomes latent (ie. the virus may or may not cause an active infection in the future).
Jumping genes - series of base pairs that excise themselves and reinsert themselves in other areas of the genome - are hypothesized as one of the ancestors of viruses. It is not unlikely that the DNA or RNA of these jumping genes couldn't have the code for some protein coating that would cover replicated strands of genetic material. These precursors could have become the first viruses.
Back to the main topic at hand. Retroviruses are specifically deadly because of their ability to hide in DNA for extremely long periods of time that allow it to permanently infect a host.
Here is why it is biologically impossible for the viruses to create "super-strength" or other super powers by happenstance... A mutation that doesn't kill a cell is 1 in a billion. A mutation that doesn't kill a cell AND benefits the cell in some way is even more rare. The likelihood that all of the muscle cells in a body receiving the same exact mutation is impossible. Not only that but every one of us has the same exact number of muscle cells. The reason we are weaker or stronger are the myosin heads and actin filaments (another story altogether). Therefore muscle cells do not reproduce and a strength-inducing mutation cannot spread through the body.
And finally, somatic cells can change all they want in a human. It won't affect your offspring. If your somatic cells suddenly are mutated and you become a grotesque insectoid monstrosity, your sperm or egg will still have human genes - assuming separate mutations didn't also (one in a million) happen to suffer the same exact mutation.
Ok, so enough of me being a party pooper. Most people don't give a heck about that. Logic and possibility mean diddly-sqwat.
If it sounds good, and isn't blatantly-insultingly-illogical to an every-day reader, then it's a fair plot device.
If you want specific and logical sounding pathways for retroviral infection, then let me know and I may be able to help.
As for symptoms, there could be a limitless list of symptoms for this. If it can affect your genome, then it could affect every function and part of your body. So have a field day with that.
An important note about DNA. DNA uses a base pair system. Many people are aware that it is double stranded and therefore T bonds with A, G with C. What fewer people know is that DNA codes for proteins in a series of 3's. Example DNA sequence... ATTTAGCAAT ... the first three letters "ATT" code for a amino acid. This code is universal for Carbon-based life on Earth. So the sequence ATTAGCAAT can be read as "ATT" "AGC" "AAT" - this sequence codes for amino acids.
Imagine a protien having 50 amino acids like this. Now insert 3 new base pairs. Consider this a mutation. The retrovirus is inserting its genome and 3 base pairs are cutting in line so to speak. You have not changed the protein drastically, as it has the same proteins, but there is an extra amino acid in the chain.
What happens if you insert 2 base pairs? You shift the entire sequence of amino acids. 2 base pairs inserted causes the entire protein and every amino acid it codes for to be different. That is called a frameshift mutation. Any addition that is not a multiple of 3 causes a frameshift mutation which results in major changes that will probably kill the cell.
But the other mutations are less dramatic. It might cause a protein to be made that interferes with a negative feedback loop that tells the cells to stop growing. In turn, a sharks tail might grow a little longer. This longer tail allows it to use the tail as a weapon to stun prey when thrashing it around... viola, you have a Thresher shark. That is essentially how evolution works, and this functional mutations.
Hope this helped or was at least interesting.
Edit: HPV - Wart virus and several STD's are very mutative viruses. Ebola, as mentioned before, is not a perfect choice. Yes it has a high mortality, but ebola affects the blood and the cell walls lining your veins. Has no more affect on your DNA than a cup of Coca Cola (probably not entirely true but it gets the point across).
As for sources of mutation - not sure a sun flare will have much affect considering sun flares happen all the time. Maybe a gamma ray burst? Most drastic mutations simply happen. Mutations that occur from everyday sunlight are will cause less drastic mutations that may be beneficial. Radiation causes mutations that are sudden and drastic, and therefore almost impossible to be beneficial.