|MadSci Network: Virology|
Well, that is certainly an interesting idea. Actually, there are no viruses more "uncurable" than any others; we cannot "cure" any viral infection. All we can do is treat the symptoms of some of them. Some are preventable with vaccines, but once you are infected, only your immune system can deal with them, sometimes successfully (like a cold), sometimes not (AIDS, Ebola).
Your question, however, is what would happen if two viruses were introduced to each other. To understand, we'll have to take a quick look at how a virus works. I'm sure there is a lengthy discussion somewhere in these archives, so I will be brief. Basically, a virus is DNA (sometimes just RNA) wrapped in a protective protein coat. All cells have the machinery needed to turn DNA into mRNA which is then "decoded" in order to produce a protein. Viruses just have the DNA or RNA, so they invade a cell and take over the cell's machinery so that the viral DNA/RNA can be decoded and viral proteins are produced. The proteins are the parts needed to make new viruses (they replicate the viral DNA and make protein coats). These viruses are then released from the cell, and go on to infect other cells. This whole process is with most viruses lethal to the cell: sometimes, because the virus has taken over the machinery, the cell can't produce what it needs to stay alive, and other times as the viruses exit the cell they explode out, destroying the cell structure. Most viruses target a certain type of cell. With hepatitis viruses, liver cells are targeted, and thus destroyed, leading to liver disease. HIV targets white blood cells, and their destruction leads to AIDS (HIV does not actually damage the body, but destroys the immune system and allows other diseases to run unchecked in the host).
With that explanation, it is relatively easy to see what would happen if two viruses were introduced to each other: basically, nothing. Inside the same cell, they would compete for machinery. The more efficient virus would produce more progeny, but it is more than likely the other virus would be able to reproduce itself at least somewhat. The net result would still be the destruction of the host cell with new virus being released. The viruses would more than likely not have any direct effect on each other.
It might be possible that the two viruses would be equal enough that in their competition for machinery they would stop each other from producing progeny, but the host cell would still wind up dead in the fight. This would, however, have the beneficial effect of no new virus being produced. Unfortunately, it would be difficult to use this to stop a deadly disease such as HIV. In your example, you suggested using a cold virus to counteract HIV. HIV, as I stated, targets white blood cells. Cold viruses target other tissues, usually in the nose. Thus, they will never be in the same cells to interfere with each other. You would have to use another white blood cell targeting virus to even try to stop HIV, and that, of course, is just pouring gasoline on the fire.
In short, because viruses are made to take over a cell's machinery to reproduce, they are not really able to affect another virus in that same cell, at least not directly. (NOTE: That being said, I must make mention of a few exceptions. There are some viruses that are unable to reproduce on their own, but can with assistance from helper viruses. For example, the hepatitis B virus (HBV) is able to infect liver cells quite well on it's own. Its cousin, the hepatitis D virus (HDV), is not-- it lacks some of the proteins it needs to take over the cell machinery. However, if HBV is in a cell at the same time HDV is then, since they are similar viruses, the HDV can "borrow" the HBV proteins and reproduce itself. On its own, HDV can't do anything, but with HBV in the same cell, it can reproduce normally. There are very few of these types of viruses, however, and they work to help each other and worsen the infection, not against each other.)
Now, since I never like to end on a disappointing note, here is my own opinion about how viruses might be used to combat viral infections, but keep in mind this is only an idea. Nowadays we can take a virus, crack it open, and change its DNA. We take out the codes that produce more virus, and put in codes that produce something we want. We stick the DNA back into the virus, and send it into a host cell. The virus takes over the cell machinery, but instead of making more virus, it would make whatever protein we told it to. We could attenuate the virus to make sure that it didn't take over too much machinery, to make sure the cell could still live. In the case of HIV, we could set up the virus to target all white blood cells. HIV is what we call a retrovirus, and one of the things it needs and produces is a protein called reverse transcriptase. The human cell does not need this protein, and so it is a target in HIV treatment (AZT, for example, inhibits reverse transcriptase function). If we could program our virus to produce a protein that inhibits reverse transcriptase, then all the white blood cells it infected would produce it. Then, when HIV shows up, its reverse transcriptase won't work, and it won't be able to take over the cell. Now, I'm not an expert on HIV, so I don't know if this would work or not; since it hasn't been done yet, I'm sure there are some complications that I don't know about that prevent this from working. Nevertheless, using a process like this, it might be possible to treat/cure a viral infection using another virus to deliver the "medicine".
I know this response was rather lengthy, but I hope I answered your question and still left a ray of hope for your idea. Viruses are fascinating organisms, and our understanding of them increases daily. I hope we will soon be able to more effectively treat viral diseases.
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